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Thread: Converting a Glasses Prescription into a Contact Lens Prescription

  1. #1

    Default Converting a Glasses Prescription into a Contact Lens Prescription

    Is there some way you can take a prescription for eye glasses and convert it into a prescription for contact lenses? I hope it's nothing too complicated. I'm just curious to know if it can be done. I've heard about the possibility, but I've never seen the formula. Any mathematicians on this board?

  2. #2
    Join Date
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    Default

    There is a formula, it is normally only an issue if you have a prescription greater than =/-4.00
    Fcl = Fsp * (1 - (d/Fsp)
    Fcl = 1/contact lens power in diopters ie -6.00= -0.167
    Fsp = 1/spectacle lens power in diopters ie -6.50= -0.154
    d = distance from the eye to the spectacle lens in meters usually 12mm = 0.012
    So an example would be, using the numbers above
    A spherical eyeglass prescription of -6.50 so

    Fcl = -0.154 * (1 - (0.012/-0.154))

    -6.02 = -0.154 * 1.078
    = -6.00
    You round to the nearest 0.25, there are charts that are much easier.
    If you have astigmatism you have to figure that in as well. If the direction of your astigmatism is say 145 there are very few contact lenses that would come in that direction so you would have to go to a 140 or a 150 even then some do not come in those directions. If you have say -1.50 of astigmatism you will usually not find any contacts that come in that power either, usually -1.25 or -1.75.
    And none of this takes into account the fit of the contact lens.
    I found the formula on Wikipedia if anyone wants to do a little recreational math, then used the charts I mentioned to work out how to get it to work.

  3. #3

    Default

    Quote Originally Posted by Sir Robin View Post
    There is a formula, it is normally only an issue if you have a prescription greater than =/-4.00
    Fcl = Fsp * (1 - (d/Fsp)
    Fcl = 1/contact lens power in diopters ie -6.00= -0.167
    Fsp = 1/spectacle lens power in diopters ie -6.50= -0.154
    d = distance from the eye to the spectacle lens in meters usually 12mm = 0.012
    So an example would be, using the numbers above
    A spherical eyeglass prescription of -6.50 so

    Fcl = -0.154 * (1 - (0.012/-0.154))

    -6.02 = -0.154 * 1.078
    = -6.00
    You round to the nearest 0.25, there are charts that are much easier.
    If you have astigmatism you have to figure that in as well. If the direction of your astigmatism is say 145 there are very few contact lenses that would come in that direction so you would have to go to a 140 or a 150 even then some do not come in those directions. If you have say -1.50 of astigmatism you will usually not find any contacts that come in that power either, usually -1.25 or -1.75.
    And none of this takes into account the fit of the contact lens.
    I found the formula on Wikipedia if anyone wants to do a little recreational math, then used the charts I mentioned to work out how to get it to work.
    "Recreational math." I love it!

    Thank you very much, Sir Robin. That is a very complete answer and I thank you for taking the time to look that up and post it here.

    Just one more question. There are some mention of rounding of numbers, such as direction of astigmatism. Is there any guideline that you know of as to whether it's best to round up or down?

  4. #4
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    Default

    The last numbers in a prescription are the axis or direction of astigmatism (ie -4.00 - 1.25 x 145) the 145 is just a direction so as far as a prescription goes, it will not change how much correction there is so there is no consistent rule as to whether you round up or down, it is something that is determined during a contact lens exam which way to "round" a direction that is not available. This is part of what is happening when given the 1 or 2 option, so when 1 and 2 look a whole lot alike it could be you are sitting on the right axis, say a 145 but only a 140 or 150 is available so that is why they are hard to tell apart. As far as rounding other numbers, I was only referring to rounding to the nearest quarter diopter, any other rounding of a prescription is determined during an exam, if necessary.

  5. #5

    Default Converting a Glasses Prescription into a Contact Lens Prescription

    Quote Originally Posted by Sir Robin View Post
    The last numbers in a prescription are the axis or direction of astigmatism (ie -4.00 - 1.25 x 145) the 145 is just a direction so as far as a prescription goes, it will not change how much correction there is so there is no consistent rule as to whether you round up or down, it is something that is determined during a contact lens exam which way to "round" a direction that is not available. This is part of what is happening when given the 1 or 2 option, so when 1 and 2 look a whole lot alike it could be you are sitting on the right axis, say a 145 but only a 140 or 150 is available so that is why they are hard to tell apart. As far as rounding other numbers, I was only referring to rounding to the nearest quarter diopter, any other rounding of a prescription is determined during an exam, if necessary.
    Thanks, Sir Robin. Hope to see you hanging around this forum in the future.

  6. #6

    Default converting spectacle rx to contact lens rx

    I didn't quite understand how to convert the rx to contact lens rx. I had a cust. who has a OD: -4.25 sph OS: -4.25 +.50 x 145 Add: +1.75 She wanted a multi-focal contact lens. How do you figure I can accomodate her. I estimated at -3.75 OS: - 3.25 OD: with hi add but I would like a formula I can consistently follow.

  7. #7

    Default

    Wow! thank you for that calculation, guys. How did you get that? Sorry but

    numbers is not really my forte. LOL!

  8. #8
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    Default Coverting from a Glasses prescription to a Contact prescription

    Please someone help me figure this out. Just got home from the eye doctor, my new glasses prescription is O.D. -650, and O.S. -550 What would that be converted to a contact prescription?? They always done it for free at the doctors before...now they want an additional $75.00 to do it. I wear Cooper Vision 43 contacts. He said, my left eye did not change any, and my right eye went from -600 up to -650. My old contact prescription is O.S. -5.75 and O.D. -6.25..... would that bump my right eye O.D. up to -7.00 Thanks for any help. r/s Dave
    Last edited by getthenet; 11-03-2009 at 05:43 PM.

  9. #9

    Default

    a patientwith -8Dspectacle,what would be the contact lens power and how to calculate it?

  10. #10
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    Quote Originally Posted by anjukutty View Post
    a patientwith -8Dspectacle,what would be the contact lens power and how to calculate it?
    Thank you Sir Robin for that calculation.

    Anjukutty

    Using Sir Robin's recreational maths formula to calculate the contact lens power, a spectacle prescription of -8.0 D would be equivalent to a contact lens prescription of -7.3 Dioptres (rounding down to -7.25 D)

    (Note: This is spherical correction only, no attempt made to correct for astigmatism)

    In my case I wear spectacle prescription of -4.25 D. The calculated contact lens prescription is with Sir Robin's formula -4.04 (-4.0 D rounded). This is what I actually use at present.

    In fact my contact lens prescription has always been -0.25 D less strong than my spectacles, so this formula seems to work OK at my prescription values.

    knotlob

  11. #11
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    Quote Originally Posted by getthenet View Post
    Please someone help me figure this out. Just got home from the eye doctor, my new glasses prescription is O.D. -650, and O.S. -550 What would that be converted to a contact prescription?? They always done it for free at the doctors before...now they want an additional $75.00 to do it. I wear Cooper Vision 43 contacts. He said, my left eye did not change any, and my right eye went from -600 up to -650. My old contact prescription is O.S. -5.75 and O.D. -6.25..... would that bump my right eye O.D. up to -7.00 Thanks for any help. r/s Dave
    Hello Dave

    I am looking at your lens data and it doesn't make complete sense to me - so are you sure you have the correct prescription data here?

    Your old prescription data is:

    Left eye (OS), Spectacles -5.50 D, Contact Lens -5.75 D. Normally in my experience the contact lens prescription is about 0.25D less than for the glasses. Sir Robin's formula for spectacles at -5.5 D would calculate a contact lens prescription of -5.16, say -5.25 D. i.e.less strong than the spectacles.

    Similarly for your right lenses:
    Right eye (OD), Spectacles -6.0 D, Contact lens -6.25 D.
    Sir Robin's formula for spectacles at -6.0 D would calculate a contact lens prescription of -5.6, say -5.5 D. i.e. again less strong than the spectacles.

    My contact lenses are normally -0.25 D less strong than my spectacles (at a prescription of around -3.5 to -4.0)

    knotlob

  12. #12

    Default Doctor's Reference Page

    Quote Originally Posted by eyezhaveit View Post
    I didn't quite understand how to convert the rx to contact lens rx. I had a cust. who has a OD: -4.25 sph OS: -4.25 +.50 x 145 Add: +1.75 She wanted a multi-focal contact lens. How do you figure I can accomodate her. I estimated at -3.75 OS: - 3.25 OD: with hi add but I would like a formula I can consistently follow.
    It looks to me like an eye doctor has come to Lens 101 for help. I hope the site was helpful.

  13. #13
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    Default

    Quote Originally Posted by anjukutty View Post
    a patientwith -8Dspectacle,what would be the contact lens power and how to calculate it?
    How was knotlob's answer? Did it help you? I'm glad he stepped in because I wouldn't know where to start.

  14. #14

    Default

    Actually most contacts manufacturers have a free calculator on their site. Your doctors just enter it online and charge you $100 for it!

    http://virtualconsultant.cibavision.com/toric_lens.jsp
    http://204.8.122.98:8888/EXEC/0/1qdysny1ikxu9710xujb40956swf

  15. #15

    Default Not Much HElp

    Quote Originally Posted by jpeg4 View Post
    Actually most contacts manufacturers have a free calculator on their site. Your doctors just enter it online and charge you $100 for it!

    http://virtualconsultant.cibavision.com/toric_lens.jsp
    http://204.8.122.98:8888/EXEC/0/1qdysny1ikxu9710xujb40956swf
    Well, it just might be worth $100 to me because the second link didn't work, and the Virtual Consultants asked for more information than I have on the card that my eye doctor gave me with my last pair of glasses. I put in the sphere, cylinder and axis and it asked me for some more values that aren't on the card. I clicked on the "calculate" button and all I got was:

    "Please enter a valid sphere value
    Please enter a valid cylinder value
    Please enter a valid rotation value
    Please enter a valid axis value
    Please enter parameters for calculation."

    It seems like it didn't like anything I did!

  16. #16

    Default

    Quote Originally Posted by Bondjamesbond View Post
    Well, it just might be worth $100 to me because the second link didn't work, and the Virtual Consultants asked for more information than I have on the card that my eye doctor gave me with my last pair of glasses. I put in the sphere, cylinder and axis and it asked me for some more values that aren't on the card. I clicked on the "calculate" button and all I got was:

    "Please enter a valid sphere value
    Please enter a valid cylinder value
    Please enter a valid rotation value
    Please enter a valid axis value
    Please enter parameters for calculation."

    It seems like it didn't like anything I did!
    Worked with me, just use the Product match: http://virtualconsultant.cibavision.com/best_match.jsp

    Click on "Convential Replacement" since they have high cylinder toric lens. Then click on calculate it'll give a list of contacts and prescriptions that fits you.

    For the second link, click on "Click here to restart Toriweb." on the bottom. Although the calculated RX doesn't match the RX my doctor gave me and I have coopervision contacts...

  17. #17

    Default Not Good With Numbers, Huh?

    Quote Originally Posted by gamblinglord View Post
    Wow! thank you for that calculation, guys. How did you get that? Sorry but

    numbers is not really my forte. LOL!
    Should a gambling lord be pretty good with numbers? Or is that the reason it's such a gamble?

    Maybe we should play some cards someday.

  18. #18

    Default I Couldn't Work It Either

    Quote Originally Posted by jpeg4 View Post
    Worked with me, just use the Product match: http://virtualconsultant.cibavision.com/best_match.jsp

    Click on "Convential Replacement" since they have high cylinder toric lens. Then click on calculate it'll give a list of contacts and prescriptions that fits you.

    For the second link, click on "Click here to restart Toriweb." on the bottom. Although the calculated RX doesn't match the RX my doctor gave me and I have coopervision contacts...
    Hi jpeg. I tried that site too, and followed your suggestions, but all I got was a phone number and an email address for some consulting company or something.

    For the second site, the toriweb one, I restarted it, and put in my numbers, but there wasn't one set of spaces for the left eye and another set for the right, just one. I plugged in my numbers for the right eye and got a recommendation of plano lenses.

    Either those sites aren't very user-friendly, or I'm just dumb as a bag of hammers.

  19. #19

    Default

    Quote Originally Posted by Eyeofthetiger View Post
    Hi jpeg. I tried that site too, and followed your suggestions, but all I got was a phone number and an email address for some consulting company or something.

    For the second site, the toriweb one, I restarted it, and put in my numbers, but there wasn't one set of spaces for the left eye and another set for the right, just one. I plugged in my numbers for the right eye and got a recommendation of plano lenses.

    Either those sites aren't very user-friendly, or I'm just dumb as a bag of hammers.
    Fill in everything exactly like mine except the OD and OS line. Same with the second link, make sure you click on both calculate buttons.

    These are made for doctors so I guess you need to know the terminologies to use it.

    Also these are for TORIC lens, if you don't have astigmatism then select 'Sphere' and leave the cylinder and axis part 0 or plano.

    http://i.imgur.com/jmXjU.png
    http://i.imgur.com/xTgBe.png

    PS: Vertex Distance is the distance between your glasses lens and your eye ball. The average is 12mm but you can use your own if you can measure it without poking out your eyeball.

  20. #20
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    Default What Happened?

    Quote Originally Posted by jpeg4 View Post
    Fill in everything exactly like mine except the OD and OS line. Same with the second link, make sure you click on both calculate buttons.

    These are made for doctors so I guess you need to know the terminologies to use it.

    Also these are for TORIC lens, if you don't have astigmatism then select 'Sphere' and leave the cylinder and axis part 0 or plano.

    http://i.imgur.com/jmXjU.png
    http://i.imgur.com/xTgBe.png

    PS: Vertex Distance is the distance between your glasses lens and your eye ball. The average is 12mm but you can use your own if you can measure it without poking out your eyeball.
    I couldn't get it to work either. I have my prescription with me, so I clicked "Disposable/Frequent Replacement" and then on the "Toric" button, copied the sphere, cylinder and axis numbers from my prescription for both eyes, put in a vertex distance of 12, no "add" power because there was none in my prescription, clicked "Recommend" and all I got for an answer was a Contact Name, Contact Number, and Email. No brand recommendations. What did I do wrong?

  21. #21

    Default

    Quote Originally Posted by KSCbase View Post
    I couldn't get it to work either. I have my prescription with me, so I clicked "Disposable/Frequent Replacement" and then on the "Toric" button, copied the sphere, cylinder and axis numbers from my prescription for both eyes, put in a vertex distance of 12, no "add" power because there was none in my prescription, clicked "Recommend" and all I got for an answer was a Contact Name, Contact Number, and Email. No brand recommendations. What did I do wrong?
    Did you put 0 for add power exactly like my picture?

    Sign... anybody want me to enter the RX for them? Somehow no one know how to operate this. Maybe that's why docs charge $100 to do it for you...

  22. #22
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    Quote Originally Posted by jpeg4 View Post
    Did you put 0 for add power exactly like my picture?

    Sign... anybody want me to enter the RX for them? Somehow no one know how to operate this. Maybe that's why docs charge $100 to do it for you...
    Both work for me OK, although it's not clear what the Coopervision calculation is supposed to do.

    knotlob

  23. #23

    Default

    Quote Originally Posted by Knotlob View Post
    Both work for me OK, although it's not clear what the Coopervision calculation is supposed to do.

    knotlob
    Yea, the coopervision one is pretty confusing. I'm guessing that it'll calculate a generic contacts RX and it'll gave you a coopervision RX. So if you have a contacts RX for another brand, you have to use it to convert it to coopervision brand RX. At least that's what I got out of it.

    But the weird thing is that I have coopervision contacts (freq. 55 toric RX) and my RX doesn't match up with the calculator. It might because my doctor gave me a generic RX. But I got near 20/20 so I'm not complaining.

  24. #24

    Default contact calculator

    regarding the http://virtualconsultant.cibavision.com/toric_lens.jsp calculator, i note several things (in fact joined here to relate those thing).

    1. it is of course imperative to remember to accurately note whether a prescription dimension is positive or negative. be certain to add a dash to the prescription if one is noted on your doctor provided prescription card.

    2. the calculator seems to function better in internet explorer (i am using IE 8 for that purpose.) it didn't seem to do so hot in chrome (the browser i normally use).

    3. while the 'Choose the type of lens' dropdown box did not initially have anything in it, after entering details and clicking the calculate button, there were items available in said dropdown.

    hope this helps! i just went to an optometrist to get my glasses lenses updated. having recently been laid off, every penny counts as i try to use my vision benefits before they are stripped from me. i am choosing to mail my frames in to have lenses fitted in them because this is much less expensive and there is no reason to not reuse the old frames. i can repair any frame damages that may occur as a result of age.

    besides that, the staff member at the optometry clinic tried telling me that the doctor would have to convert my script for me (at an $80 charge, of course) and that it was required to do some extra special tests to make sure the contacts would be ok. i smell a rat.

  25. #25

    Default Contact Rx

    I went to the doctor yesterday for my exam and my eyes had gotten a little worse this past year, so when I got ready to leave I saw that they had forgotten to give me my trial pair to wear for a week just to try out and see how they work. I went back to the counter to tell the girl they had forgotten to give them to me, and she opened my chart and said well the doctor left to get a coke but I will look at what he wrote and give them to you, so when I get home I see they are a weaker prescription than I had last year. My prescription for my glasses are stronger, and when I put my contacts in my vision is blurry and I'm wondering if they gave me the wrong thing. My Rx for my glasses is: (R) +125/-050/086, (L) +100/-050/108 and the contacts they gave me were (R) +100 (L) +0.75. Could anyone help of give and suggestions?

  26. #26
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    Quote Originally Posted by mbalch73 View Post
    I went to the doctor yesterday for my exam and my eyes had gotten a little worse this past year, so when I got ready to leave I saw that they had forgotten to give me my trial pair to wear for a week just to try out and see how they work. I went back to the counter to tell the girl they had forgotten to give them to me, and she opened my chart and said well the doctor left to get a coke but I will look at what he wrote and give them to you, so when I get home I see they are a weaker prescription than I had last year. My prescription for my glasses are stronger, and when I put my contacts in my vision is blurry and I'm wondering if they gave me the wrong thing. My Rx for my glasses is: (R) +125/-050/086, (L) +100/-050/108 and the contacts they gave me were (R) +100 (L) +0.75. Could anyone help of give and suggestions?
    Hello mbalch73

    Welcome to the forum.

    You quote an Rx for your spectacles of +125/-050/086 (Right Lens) and +100/-050/108 (Left Lens). I guess what you really mean is:

    Right Lens: +1.25D/-0.5D/080 Deg
    Left Lens: +1.00D/-0.5D/108 Deg
    (D is Dioptres = the Power of the Lens)
    The first figure is the Spherical Correction
    The second figure is the Astigmatism Cylinder Power
    The Third figure is the Axis or Cylinder Angle

    Your Contact lenses are:
    Right lens: +1.00D
    Left Lens (I assume): +0.75D
    (i.e. no correction for astigmatism)

    The spectacles will usually be slightly different in power to the contacts, especially as your correction power increases, due to the distance between your cornea and the correction lens, zero for contact lenses and usually 12mm approximately for spectacles.

    Sir Robin, on this forum, previously posted a formula to convert spectacle prescription to Contact Lens prescription (Refractive Correction) and the values for your Contact lens prescription would be (assuming a distance between your cornea and the spectacle lens of 0.012m, i.e. 12mm):

    Right Lens: +1.27D (say 1.25D)
    for Left Lens: +1.01D (say +1.0D)

    When there is an astigmatism correction not corrected by the contact lenses, the doctor may add (or effectively subtract in your case since you have positive spherical correction) half of it's value from the contact spherical correction. In your case half of -0.50D astigmatism equals -0.25D.

    So in this case, it looks like the relationship between the spectacles and the contact lenses is correct. My optician normally checks the contact lenses for fit and vision accuracy after one week or so. It may be you just need to give the contacts more time to settle to your eye or maybe a bit of fine tuning is required to the strength of the contact lenses.

    I am assuming that you did not wear your contacts for say 24hours immediately before your eye examination, unless the optician requested otherwise.

    knotlob

  27. #27

    Default Contacts and Eyeglass prescription difference.

    Hey everyone,

    Wondering if someone could help me out. I'm thoroughly confused about something.

    Contact lens prescription:

    Right eye: -3.25
    Left eye: -5.50

    Eyeglass prescription:

    Right eye: -3.50
    Left eye: -6.25

    I'm wondering why there seems to be such a large difference between the contact and eyeglass prescription for my LEFT eye, whereas the right eye difference seems to make more sense to me.

    A little history: (I'll try to make it as brief as I can )

    I'd been in for a VERY thorough eye check-up a year ago - actually went in for a contact lens fitting, and doc wrote out the eyeglass prescription too.

    We moved shortly after, I broke my old frames and now need new ones. I got my prescription faxed to me and used it to order new glasses online (had previously ordered contacts from the same website and was very happy with the quality of service and price).

    My glasses arrived today and I'm worried that the left lens might be too powerful? It feels a little 'magnified', if that makes sense. Then again, I'm not sure if my eyes need time to get used to it?

    When I checked the eyeglass prescription against the contacts prescription, I was surprised to see the difference for the left eye. I've heard it should typically be a difference of about .25 to .50.

    Should I can send these glasses back for a new pair, with the left lens at maybe 6.00 or 5.75? I know the formula posted here is for eyeglass to contact conversion, and I'm a little too mathematically challenged to reverse the formula

    Either way, when I did the conversion based on my 6.25 left eye glasses prescription, my contact prescr. comes to -5.81. Doesn't that imply my left eye contact lens should be -5.75 and not -5.50?

    (FYI - I can see perfectly with my -5.50 contact lens)

    Any help would be HUGELY appreciated. THANK YOU!

  28. #28
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    Default Contacts and Eyeglass Prescription Difference

    Quote Originally Posted by MariaCL View Post
    Hey everyone,

    Wondering if someone could help me out. I'm thoroughly confused about something.

    Contact lens prescription:

    Right eye: -3.25
    Left eye: -5.50

    Eyeglass prescription:

    Right eye: -3.50
    Left eye: -6.25

    I'm wondering why there seems to be such a large difference between the contact and eyeglass prescription for my LEFT eye, whereas the right eye difference seems to make more sense to me.

    A little history: (I'll try to make it as brief as I can )

    I'd been in for a VERY thorough eye check-up a year ago - actually went in for a contact lens fitting, and doc wrote out the eyeglass prescription too.

    We moved shortly after, I broke my old frames and now need new ones. I got my prescription faxed to me and used it to order new glasses online (had previously ordered contacts from the same website and was very happy with the quality of service and price).

    My glasses arrived today and I'm worried that the left lens might be too powerful? It feels a little 'magnified', if that makes sense. Then again, I'm not sure if my eyes need time to get used to it?

    When I checked the eyeglass prescription against the contacts prescription, I was surprised to see the difference for the left eye. I've heard it should typically be a difference of about .25 to .50.

    Should I can send these glasses back for a new pair, with the left lens at maybe 6.00 or 5.75? I know the formula posted here is for eyeglass to contact conversion, and I'm a little too mathematically challenged to reverse the formula

    Either way, when I did the conversion based on my 6.25 left eye glasses prescription, my contact prescr. comes to -5.81. Doesn't that imply my left eye contact lens should be -5.75 and not -5.50?

    (FYI - I can see perfectly with my -5.50 contact lens)

    Any help would be HUGELY appreciated. THANK YOU!
    Hi Maria. I see you've found our little website here. Welcome to Lens 101. What kind (brand) of contacts are you being fitted for?

    I'm afraid I can do little more than greet you and make you feel comfortable. I can tell you that contact lens prescriptions are different from eyeglasses prescriptions because the contact lens sits directly on your cornea while glasses are an inch or two in front of it.
    I don't know why in your case one side is more different than the other side. You may want to get a second opinion from another eye doctor. While you're there explaining why you'd like to be fitted for contacts again, the doctor may be able to explain the discrepancy.
    I know that wasn't much help, but I'm just a Regular Guy trying to help where I can. I'm sure some of my colleagues here on Lens 101 can give you a more satisfactory answer.
    Last edited by Orionebula; 04-05-2010 at 08:56 AM.

  29. #29
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    Default

    Quote Originally Posted by MariaCL View Post
    Hey everyone,

    Wondering if someone could help me out. I'm thoroughly confused about something.

    Contact lens prescription:

    Right eye: -3.25
    Left eye: -5.50

    Eyeglass prescription:

    Right eye: -3.50
    Left eye: -6.25

    I'm wondering why there seems to be such a large difference between the contact and eyeglass prescription for my LEFT eye, whereas the right eye difference seems to make more sense to me.

    A little history: (I'll try to make it as brief as I can )

    I'd been in for a VERY thorough eye check-up a year ago - actually went in for a contact lens fitting, and doc wrote out the eyeglass prescription too.

    We moved shortly after, I broke my old frames and now need new ones. I got my prescription faxed to me and used it to order new glasses online (had previously ordered contacts from the same website and was very happy with the quality of service and price).

    My glasses arrived today and I'm worried that the left lens might be too powerful? It feels a little 'magnified', if that makes sense. Then again, I'm not sure if my eyes need time to get used to it?

    When I checked the eyeglass prescription against the contacts prescription, I was surprised to see the difference for the left eye. I've heard it should typically be a difference of about .25 to .50.

    Should I can send these glasses back for a new pair, with the left lens at maybe 6.00 or 5.75? I know the formula posted here is for eyeglass to contact conversion, and I'm a little too mathematically challenged to reverse the formula

    Either way, when I did the conversion based on my 6.25 left eye glasses prescription, my contact prescr. comes to -5.81. Doesn't that imply my left eye contact lens should be -5.75 and not -5.50?

    (FYI - I can see perfectly with my -5.50 contact lens)

    Any help would be HUGELY appreciated. THANK YOU!
    Hi MariaCL, welcome to Lens101.

    Using Sir Robin's formula (2nd post in this thread) I calculate that with a Spectacle prescription of -3.50, your contact lens prescription would be -3.36D. So you optician has rounded down to -3.25D.

    For your left eye, your spectacle prescription is -6.25D, I calculate the contact lens prescription should be -5.81D, so your optician would normally round down to say -5.75D. In your case, you have been prescribed a contact lens power of -5.50D. Now you don't say what brand of contact lenses you are wearing, but perhaps they are not available in a prescription of -5.75D ??

    Using the formula, you will see that at low powers (say -1.5D) there is precious little difference between a spectacle prescription and a contact lens prescription. But as your prescription power increases, the difference between the spectacle and contact lens prescription increases, so explaining why your left eye contact lens prescription is more different than your right eye, versus the spectacle correction.

    I think normally the eye care specialist will measure your spectacle prescription first and then apply the correction to get the contact lens prescription power. There are more complications associated with fitting contact lenses than for spectacles.

    Maybe you should check on line to see if your prescription for your left contact lens is available in -5.75D and if so, you can query why your optician specified -5.50D. However, you say that your eyesight is fine with the -5.50D contact lens, so if it ain't broke, don't fix it!

    I would give the glasses a bit more time to settle down. If you don't get any headaches, they will probably be OK.

    knotlob

  30. #30

    Default

    Thanks for the replies!! I'm going to wear my glasses for a while and see if it gets better.

    I read somewhere that the 'fishbowl' vision that comes with new eyeglasses is generally due to lack of proper centering? Any truth to that?

  31. #31

    Default

    Quote Originally Posted by MariaCL View Post
    Thanks for the replies!! I'm going to wear my glasses for a while and see if it gets better.

    I read somewhere that the 'fishbowl' vision that comes with new eyeglasses is generally due to lack of proper centering? Any truth to that?
    Nah, it's always gonna be there but after a while you will get use to it.

  32. #32
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    Default Told You

    Quote Originally Posted by Knotlob View Post
    Hi MariaCL, welcome to Lens101.

    Using Sir Robin's formula (2nd post in this thread) I calculate that with a Spectacle prescription of -3.50, your contact lens prescription would be -3.36D. So you optician has rounded down to -3.25D.

    For your left eye, your spectacle prescription is -6.25D, I calculate the contact lens prescription should be -5.81D, so your optician would normally round down to say -5.75D. In your case, you have been prescribed a contact lens power of -5.50D. Now you don't say what brand of contact lenses you are wearing, but perhaps they are not available in a prescription of -5.75D ??

    Using the formula, you will see that at low powers (say -1.5D) there is precious little difference between a spectacle prescription and a contact lens prescription. But as your prescription power increases, the difference between the spectacle and contact lens prescription increases, so explaining why your left eye contact lens prescription is more different than your right eye, versus the spectacle correction.

    I think normally the eye care specialist will measure your spectacle prescription first and then apply the correction to get the contact lens prescription power. There are more complications associated with fitting contact lenses than for spectacles.

    Maybe you should check on line to see if your prescription for your left contact lens is available in -5.75D and if so, you can query why your optician specified -5.50D. However, you say that your eyesight is fine with the -5.50D contact lens, so if it ain't broke, don't fix it!

    I would give the glasses a bit more time to settle down. If you don't get any headaches, they will probably be OK.

    knotlob
    See? I told you someone would come along and improve on my floundering for an answer.

  33. #33

    Default

    Hi Everyone! I'm so glad i found this website! I have a question about my Eyeglass prescription,i do have astigmatism and when i got my prescription, i didn't get it for my contacts,can someone please,please help!I am definitely not good with numbers,probably worse than others Well here is my prescription R -1.50 -3.00(cylinder) 180(axis)
    L -1.25 -2.50(cylinder) 173(axis)
    Thanks Everyone!

  34. #34

    Default I'd Like to Buy a Calculator, Please

    Quote Originally Posted by ImJustNotSure View Post
    Hi Everyone! I'm so glad i found this website! I have a question about my Eyeglass prescription,i do have astigmatism and when i got my prescription, i didn't get it for my contacts,can someone please,please help!I am definitely not good with numbers,probably worse than others Well here is my prescription R -1.50 -3.00(cylinder) 180(axis)
    L -1.25 -2.50(cylinder) 173(axis)
    Thanks Everyone!
    Hello ImJustNotSure,

    Here is Sir Robin's Magic Formula from the top of this post. Unfortunately it only works to calculate the sphere. Let me see if I can plug in the numbers for you. Please understand that I typically rely on my good looks to cover my mathematical shortcomings. (If you could see me, you might be tempted to buy me a calculator.) Here we go.

    Fcl = Fsp * (1 - (d/Fsp)

    Fcl = 1/contact lens power in diopters ie -6.00= -0.167
    Fsp = 1/spectacle lens power in diopters ie -6.50= -0.154
    d = distance from the eye to the spectacle lens in meters usually 12mm = 0.012

    Right eye:

    Fsp = 1/-1.50=-0.5

    Fcl = -0.5 *(1-.012/-0.5)
    1-.012= .998

    Fcl = -0.5 *.998 = .488


    Bah! That's not right. Try this formula:

    Call your eye doctor + ask what your contact lens prescription would be = contact lens prescription.

  35. #35
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    Default

    Quote Originally Posted by bettydaviseyes View Post
    Hello ImJustNotSure,

    Here is Sir Robin's Magic Formula from the top of this post. Unfortunately it only works to calculate the sphere. Let me see if I can plug in the numbers for you. Please understand that I typically rely on my good looks to cover my mathematical shortcomings. (If you could see me, you might be tempted to buy me a calculator.) Here we go.

    Fcl = Fsp * (1 - (d/Fsp)

    Fcl = 1/contact lens power in diopters ie -6.00= -0.167
    Fsp = 1/spectacle lens power in diopters ie -6.50= -0.154
    d = distance from the eye to the spectacle lens in meters usually 12mm = 0.012

    Right eye:

    Fsp = 1/-1.50=-0.5

    Fcl = -0.5 *(1-.012/-0.5)
    1-.012= .998

    Fcl = -0.5 *.998 = .488


    Bah! That's not right. Try this formula:

    Call your eye doctor + ask what your contact lens prescription would be = contact lens prescription.
    You need to be careful with these brackets!

    I have this formula on a spreadsheet, so:

    Right spectacle lens is -1.5D
    equates to Right Contact lens value (spherical correction only) -1.47
    (i.e. very little difference at these low spherical corrections)

    Left Spectacle Contact Lens -1.25
    equates to Left Contact lens value of -1.23
    (again very little difference due to Vertex correction)

    But this case is complicated by the astigmatism of -3.0D and -2.5D for right and left lenses respectively. As I understand it, the optician will normally add half the astigmatism value to the spherical correction value (where the contact lens has no astigmatism correction). So your lenses could be -1.5D -1.5D = -3.0D for the right lens. This 'artificial' astigmatism correction is not going to work.

    The -3.0D astigmatism is quite severe and you may need specialist order or Extended Range Toric Contact lenses to correct the serious astigmatism.

    So, in short, you need to see your optician and allow them to specify a suitable toric contact lens for your eyes. You cannot guess or calculate your contact lens prescription very easily in this case, if you want to have the best eye sight prescription for your own case.

    knotlob

  36. #36

    Default Knotlob to the Rescue -- Again

    Quote Originally Posted by Knotlob View Post
    You need to be careful with these brackets!

    I have this formula on a spreadsheet, so:

    Right spectacle lens is -1.5D
    equates to Right Contact lens value (spherical correction only) -1.47
    (i.e. very little difference at these low spherical corrections)

    Left Spectacle Contact Lens -1.25
    equates to Left Contact lens value of -1.23
    (again very little difference due to Vertex correction)

    But this case is complicated by the astigmatism of -3.0D and -2.5D for right and left lenses respectively. As I understand it, the optician will normally add half the astigmatism value to the spherical correction value (where the contact lens has no astigmatism correction). So your lenses could be -1.5D -1.5D = -3.0D for the right lens. This 'artificial' astigmatism correction is not going to work.

    The -3.0D astigmatism is quite severe and you may need specialist order or Extended Range Toric Contact lenses to correct the serious astigmatism.

    So, in short, you need to see your optician and allow them to specify a suitable toric contact lens for your eyes. You cannot guess or calculate your contact lens prescription very easily in this case, if you want to have the best eye sight prescription for your own case.

    knotlob
    Thanks for coming to the rescue, Knotlob. Bettydaviseyes got me all confused. I'm not that great in math either, so I doubt if I could have done any better. I liked that final calculation that involved calling your eye doctor. That makes more sense to me. However, it's probably cheaper to ask Lens 101, right?

  37. #37

    Default

    Thanks For Your Help and Info knotlob! I will definitely check up!

  38. #38
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    Default

    Quote Originally Posted by Eldradmustlive View Post
    Thanks for coming to the rescue, Knotlob. Bettydaviseyes got me all confused. I'm not that great in math either, so I doubt if I could have done any better. I liked that final calculation that involved calling your eye doctor. That makes more sense to me. However, it's probably cheaper to ask Lens 101, right?
    Yeah, the astigmatism correction is a little complicated. It is covered in this on-line tutorial:

    http://www.eyetec.net/group7/M34S1.htm

    knotlob

  39. #39

    Default

    hey i`m just wondering if you could help me convert my glasses prescriptions to contact lenses prescription ?
    ive been getting different answers and im confused
    HAHAHAHA likeeee ,
    my friend says they both have to be the same degree or something
    ANYWAY
    my right eye is -350/-050x95
    my left eye is -275
    THANKS !

  40. #40
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    Default

    Quote Originally Posted by __s2__ View Post
    hey i`m just wondering if you could help me convert my glasses prescriptions to contact lenses prescription ?
    ive been getting different answers and im confused
    HAHAHAHA likeeee ,
    my friend says they both have to be the same degree or something
    ANYWAY
    my right eye is -350/-050x95
    my left eye is -275
    THANKS !
    Hello __s2__ and welcome to the forum

    This formula of Sir Robin's is for Spherical correction only and not for astigmatism. However, in your case: Right Eye:

    Contact lens: -3.36D (for a spectacle of -3.5D). You have astigmatism in the right eye of -0.50, so assuming that your optician did not opt for toric lenses which are more troublesome and expensive, he/she may add half of the astigmatism correction to your spherical correction. So the lens would be (-3.36) - (-0.25) = -3.61 D. They would have to round this correction up or down to the nearest available contact lens and usually they go the slightly weaker correction initially. So -3.50D, or alternatively rounding up, -3.75D (all for a non toric lens).

    Left eye -2.75D (Spectacles) and no astigmatism. Contact lens prescription -2.66D. Rounding down to nearest contact lens would give a prescription of -2.50D or rounding up -2.75D.

    These are indicative values only. The optician would use such a calculation to select a lens to try as a first pass and then check your eye sight with these in your eyes. More importantly, they would have to examine your eyes to see that they are healthy and suitable for contact lenses. They would also have to select and check a contact lens base curve for a good fit - i.e. one that allows the lens to float properly on your cornea to ensure a proper tear film, etc.

    This is not something you can do yourself at home.

    knotlob

  41. #41

    Default

    Quote Originally Posted by Knotlob View Post
    Yeah, the astigmatism correction is a little complicated. It is covered in this on-line tutorial:

    http://www.eyetec.net/group7/M34S1.htm

    knotlob
    I went to that website and it's very interesting. Thanks for the link, knotlob.

    Now don't get lost at that other site everybody. Leave a trail of bread crumbs so you can come back to Lens 101.
    Attached Images Attached Images

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    Default

    Quote Originally Posted by Bondjamesbond View Post
    "Recreational math." I love it!

    Thank you very much, Sir Robin. That is a very complete answer and I thank you for taking the time to look that up and post it here.

    Just one more question. There are some mention of rounding of numbers, such as direction of astigmatism. Is there any guideline that you know of as to whether it's best to round up or down?
    Recreational math? What's next? Casual physics? Amateur biology?

  43. #43

    Default

    Quote Originally Posted by Bondjamesbond View Post
    I went to that website and it's very interesting. Thanks for the link, knotlob.

    Now don't get lost at that other site everybody. Leave a trail of bread crumbs so you can come back to Lens 101.
    Yeah, it's easy to get lost on the Internet. You never know what you'll find.
    Attached Images Attached Images

  44. #44
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    Default

    Quote Originally Posted by Sir Robin View Post
    There is a formula, it is normally only an issue if you have a prescription greater than =/-4.00
    Fcl = Fsp * (1 - (d/Fsp)
    Fcl = 1/contact lens power in diopters ie -6.00= -0.167
    Fsp = 1/spectacle lens power in diopters ie -6.50= -0.154
    d = distance from the eye to the spectacle lens in meters usually 12mm = 0.012
    So an example would be, using the numbers above
    A spherical eyeglass prescription of -6.50 so

    Fcl = -0.154 * (1 - (0.012/-0.154))

    -6.02 = -0.154 * 1.078
    = -6.00
    You round to the nearest 0.25, there are charts that are much easier.
    If you have astigmatism you have to figure that in as well. If the direction of your astigmatism is say 145 there are very few contact lenses that would come in that direction so you would have to go to a 140 or a 150 even then some do not come in those directions. If you have say -1.50 of astigmatism you will usually not find any contacts that come in that power either, usually -1.25 or -1.75.
    And none of this takes into account the fit of the contact lens.
    I found the formula on Wikipedia if anyone wants to do a little recreational math, then used the charts I mentioned to work out how to get it to work.
    That seems kinda complicated to me. I suppose that's as simple as it gets, isn't it?

  45. #45
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    Default I'm Lost

    Quote Originally Posted by Eldradmustlive View Post
    Yeah, it's easy to get lost on the Internet. You never know what you'll find.


    That's a very good illustration of how it feels sometimes. You look around and go "How did I get here?"

  46. #46

    Default help

    Hi,

    This is my first post here and i was wondering if you guys can help.

    I have my glasses prescription but have absoultly no idea how to get the POW needed to order my contacts. I'm guessing you need only one number/POW for each eye, and i have this bunch of numbers. I tried 'googleing' and didn't help much either except figuring out what all the abbreviations mean....

    that is until I saw this post and it looked like alot of help, but... too bad my understanding of all this is not too good. SO i was wondering if somebody could help. So, this is pretty much what my pres' says:
    .................................................. ....

    Right........................Left
    -2.00........SPH..........-2.00
    -0.25........CYL..........+0.00
    79*..........AXS...........0*
    I 0.00.......PSM..........O 0.00
    U 0.00......................U 0.00

    *these numberes are in degrees
    .................................................. .....


    so, could somebody (more experienced than i am ) find the POW of my current lenses and a reccomendation for my contact strength

    thank you.

  47. #47
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    Default

    Quote Originally Posted by zav View Post
    Hi,

    This is my first post here and i was wondering if you guys can help.

    I have my glasses prescription but have absoultly no idea how to get the POW needed to order my contacts. I'm guessing you need only one number/POW for each eye, and i have this bunch of numbers. I tried 'googleing' and didn't help much either except figuring out what all the abbreviations mean....

    that is until I saw this post and it looked like alot of help, but... too bad my understanding of all this is not too good. SO i was wondering if somebody could help. So, this is pretty much what my pres' says:
    .................................................. ....

    Right........................Left
    -2.00........SPH..........-2.00
    -0.25........CYL..........+0.00
    79*..........AXS...........0*
    I 0.00.......PSM..........O 0.00
    U 0.00......................U 0.00

    *these numberes are in degrees
    .................................................. .....


    so, could somebody (more experienced than i am ) find the POW of my current lenses and a reccomendation for my contact strength

    thank you.
    Hello zav

    Welcome to the forum.

    I will explain what these numbers mean to you, but you cannot just go and buy a pair of contact lenses on-line. There is much more to it than that. You must have your eyes checked specifically for contact lenses and when you try a pair, the eye care professional (ECP) will see how the lens floats on your eye. It should move off slightly and then recentre as you blink.

    The ECP will also teach you how to put the lenses in, how to take them out and how to clean them and generally look after them and the case if you are not wearing daily disposable lenses. They will also advise you how long you should wear the lenses for and also that you should come back for a check annually to see if your eyes are still OK with the lenses.

    You can do a lot of damage to your eyes wearing contact lenses improperly (even to the loss of eyesight!).

    OK, Right Eye.
    Power is the spherical correction needed with glasses to see properly at (normally) long distance. In your case it is -2.00D with spectacles.
    The second figure of -0.25D is the Cylinder correction for astigmatism and the Axis is 70 Deg which is how the cylinder correction is to be aligned in your eye. Your astigmatism is tiny so would not normally be corrected with contact lenses.

    I don't know what I, O, U are unless they are to do with 'Near' sight - normally called ADD, but since they are zero, you can ignore them.

    The left eye has no astigmatism, just a simple spherical correction of -2.00D.

    So plugging the numbers into my spreadsheet, the contact lens spherical correction will be:

    -1.95D in both cases. i.e. your contact lens power will be -2.00D. There would normally be a slight spherical correction (half of the astigmatism correction) applied to the spherical lens in your right eye, but this is too small to bother about, so it is a standard soft contact lens of -2.00D with no astigmatism correction.

    But as I have said, you need to go to your ECP and have them fit the contact lenses, otherwise you may be back here posting that you have damaged your eyes in a few weeks time!

    knotlob

  48. #48

    Default

    Quote Originally Posted by Knotlob View Post
    Hello zav

    Welcome to the forum.

    I will explain what these numbers mean to you, but you cannot just go and buy a pair of contact lenses on-line. There is much more to it than that. You must have your eyes checked specifically for contact lenses and when you try a pair, the eye care professional (ECP) will see how the lens floats on your eye. It should move off slightly and then recentre as you blink.

    The ECP will also teach you how to put the lenses in, how to take them out and how to clean them and generally look after them and the case if you are not wearing daily disposable lenses. They will also advise you how long you should wear the lenses for and also that you should come back for a check annually to see if your eyes are still OK with the lenses.

    You can do a lot of damage to your eyes wearing contact lenses improperly (even to the loss of eyesight!).

    OK, Right Eye.
    Power is the spherical correction needed with glasses to see properly at (normally) long distance. In your case it is -2.00D with spectacles.
    The second figure of -0.25D is the Cylinder correction for astigmatism and the Axis is 70 Deg which is how the cylinder correction is to be aligned in your eye. Your astigmatism is tiny so would not normally be corrected with contact lenses.

    I don't know what I, O, U are unless they are to do with 'Near' sight - normally called ADD, but since they are zero, you can ignore them.

    The left eye has no astigmatism, just a simple spherical correction of -2.00D.

    So plugging the numbers into my spreadsheet, the contact lens spherical correction will be:

    -1.95D in both cases. i.e. your contact lens power will be -2.00D. There would normally be a slight spherical correction (half of the astigmatism correction) applied to the spherical lens in your right eye, but this is too small to bother about, so it is a standard soft contact lens of -2.00D with no astigmatism correction.

    But as I have said, you need to go to your ECP and have them fit the contact lenses, otherwise you may be back here posting that you have damaged your eyes in a few weeks time!

    knotlob

    thankyou

    i'v had contacts before, so i i know how to put them in and all the other info, risks ect., just that this time around, i was lost on reading the prescription sheet!

    thank you once again

    appreciate it!

  49. #49
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    Default What Are All These Numbers?

    Quote Originally Posted by zav View Post
    thankyou

    i'v had contacts before, so i i know how to put them in and all the other info, risks ect., just that this time around, i was lost on reading the prescription sheet!

    thank you once again

    appreciate it!
    Oh, so you're just curious about what the numbers mean? It's a good thing that knotlob answered your question. I don't think anyone could have explained it as well. He should be given some sort of Valued Member status or something because he knows so much about contact lenses.

    By the way, maybe the IOU numbers on the prescription are your outstanding balance with your eye doctor. If it was all zeros, that would be good news.

  50. #50

    Smile ugh my head is spinning!!

    Im sorry Im so bad with numbers and calculations!! Can someone please do this one for me?? I have astigmatism in both eyes and need to convert to contacts. Heres my glasses Rx:

    OD~ -0.25-1.25x137
    OS~ -0.50-1.00x018

    I would really appreciate if someone could figure this one out for me

  51. #51

    Default

    Quote Originally Posted by crxchicky View Post
    Im sorry Im so bad with numbers and calculations!! Can someone please do this one for me?? I have astigmatism in both eyes and need to convert to contacts. Heres my glasses Rx:

    OD~ -0.25-1.25x137
    OS~ -0.50-1.00x018

    I would really appreciate if someone could figure this one out for me
    Hi crxchicky. I'm probably as bad with calculations as you are, so I'm going to back away slowly. Hopefully someone else more competent will come along. See ya.

  52. #52

    Default

    Thanks CousinMuscles:

    I hope someone comes along that can help. I also wanted to add that I have had contacts b4. I actually called my pharmacy for the prescription for the contacts but bc they had them as trial pair, she couldnt see the persciption. So I dont exactly want to go back in there bc they WILL charge me for a sitting fee. I know my prescription is still working and I dont need a new one... grr...
    I usually used Acuvue Oasis for Astigmatism.
    Theres no conversion tables online eh?

  53. #53
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    Default Semi-Helpful Site

    Quote Originally Posted by crxchicky View Post
    Thanks CousinMuscles:

    I hope someone comes along that can help. I also wanted to add that I have had contacts b4. I actually called my pharmacy for the prescription for the contacts but bc they had them as trial pair, she couldnt see the persciption. So I dont exactly want to go back in there bc they WILL charge me for a sitting fee. I know my prescription is still working and I dont need a new one... grr...
    I usually used Acuvue Oasis for Astigmatism.
    Theres no conversion tables online eh?
    That's a great suggestion. I looked on line for a conversion table, and instead I found a site that will automatically do the calculations for you: http://www.numericalexample.com/content/view/32/33

    The glasses prescription I have has my right eye at -1.75. According to the site, that translates to a contact lens power of -1.71. Pretty much the same.

    The only reservation I have is that I had to guess my values for "type of glasses" and "extra distance."
    The drop down menu for "type of glasses" gives you the options of "contact lenses," "glasses 12 mm," "glasses 14mm" and "other." I'm not 100% sure, but that sounds like the distance between your eye and the glasses lens.

    Check out the site and see what you think.

  54. #54
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    Default

    Quote Originally Posted by crxchicky View Post
    Im sorry Im so bad with numbers and calculations!! Can someone please do this one for me?? I have astigmatism in both eyes and need to convert to contacts. Heres my glasses Rx:

    OD~ -0.25-1.25x137
    OS~ -0.50-1.00x018

    I would really appreciate if someone could figure this one out for me
    Hello crxchicky

    The normal distance between your spectacle lenses and your cornea is about 12mm. So putting your spherical spectacle correction into Sir Robin's formula gives a prescription for contact lenses exactly the same as for glasses. Normally for prescriptions of strength less than -4.00D, there is very little difference between glasses and contact lenses. Your spherical correction is extremely minor, hence the prescriptions for glasses and contact lenses are the same.

    I am assuming that you will be wearing toric contact lenses to correct your astigmatism.

    knotlob

  55. #55

    Default thank you

    Quote Originally Posted by Knotlob View Post
    Hello crxchicky

    The normal distance between your spectacle lenses and your cornea is about 12mm. So putting your spherical spectacle correction into Sir Robin's formula gives a prescription for contact lenses exactly the same as for glasses. Normally for prescriptions of strength less than -4.00D, there is very little difference between glasses and contact lenses. Your spherical correction is extremely minor, hence the prescriptions for glasses and contact lenses are the same.

    I am assuming that you will be wearing toric contact lenses to correct your astigmatism.

    knotlob


    Thanks Knotlob
    Im trying to order my contacts online tho and it only gives u the option for axis in multiples of 10?
    So when i order them...
    OD being - 0.25-1.25x137 ~ would that mean i should order -0.25-1.25x140?
    OS being -0.50-1.00x018 ~ would that mean i should order -0.50-1.00x020?

    also i just checked but the left eye... does not give me the option of -1.00.. the option is -0.75,-1.25,-1.75,or -2.25; so do i make it -0.50-0.75x020?
    sorry if that was confusing...

    Thanks for coping with my stupidity.. i just really dont want to order wrong

  56. #56
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    Default

    Quote Originally Posted by crxchicky View Post
    Thanks Knotlob
    Im trying to order my contacts online tho and it only gives u the option for axis in multiples of 10?
    So when i order them...
    OD being - 0.25-1.25x137 ~ would that mean i should order -0.25-1.25x140?
    OS being -0.50-1.00x018 ~ would that mean i should order -0.50-1.00x020?

    also i just checked but the left eye... does not give me the option of -1.00.. the option is -0.75,-1.25,-1.75,or -2.25; so do i make it -0.50-0.75x020?
    sorry if that was confusing...

    Thanks for coping with my stupidity.. i just really dont want to order wrong
    The axis values of 137 is close enough to 140 deg for the OD (right eye) and the 18 deg close enough to the 20 deg for the (left eye) OS.

    It really is an optician's decision whether to go for a higher or lower power on the astigmatism. However, they would normally try the lower power (-0.75D) cylinder for your left eye first.

    The advantage of going to an eye care specialist is that if the lenses are not quite right, they can supply you with a modified prescription which fits better and also sort out any other problems that may arise.

    knotlob

  57. #57

    Default How Close?

    Quote Originally Posted by Knotlob View Post
    The axis values of 137 is close enough to 140 deg for the OD (right eye) and the 18 deg close enough to the 20 deg for the (left eye) OS.

    knotlob
    For those of us on Lens 101 trying to make these calculations at home, how close is "close enough"? In the example above, three (diopters?) is the biggest separation that seems to be "close enough."

  58. #58
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    Quote Originally Posted by Raidernation11 View Post
    For those of us on Lens 101 trying to make these calculations at home, how close is "close enough"? In the example above, three (diopters?) is the biggest separation that seems to be "close enough."
    No, not dioptres. The 137 and 140 Degrees are the axis measurements so I said they were close enough. The difference in cylinder power for the left lens was only 0.25 Dioptres.

    knotlob

  59. #59

    Default awesome!!

    Quote Originally Posted by Knotlob View Post
    The axis values of 137 is close enough to 140 deg for the OD (right eye) and the 18 deg close enough to the 20 deg for the (left eye) OS.

    It really is an optician's decision whether to go for a higher or lower power on the astigmatism. However, they would normally try the lower power (-0.75D) cylinder for your left eye first.

    The advantage of going to an eye care specialist is that if the lenses are not quite right, they can supply you with a modified prescription which fits better and also sort out any other problems that may arise.

    knotlob
    Thanks so much knotlob...
    I think Im ready to order

  60. #60

    Default Mission Accomplished

    Quote Originally Posted by crxchicky View Post
    Thanks so much knotlob...
    I think Im ready to order
    Fantastic. I'm glad Lens 101 could help you. That knotlob is great to have around. Spread the word.

    Let us know how you like the contacts when you get them.

  61. #61

    Default Can someone help me convert my glasses prescription into contacts?

    I want to start wearing prescription circle lenses but I don't know the power of contact lenses. I only have my glasses prescription.
    OD: -175
    OS: -175

    OD CYL: spl
    OS CYL: spl

    Then in my axis and prism and etc. They're just blank, nothing written in the column. In the ADD column theres nothing written in them either. That's all the information I was given on my glasses prescription. -175 in both eyes.
    Can anyone help change it into contacts? I think it should be fairly easy...

  62. #62
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    Quote Originally Posted by asian View Post
    I want to start wearing prescription circle lenses but I don't know the power of contact lenses. I only have my glasses prescription.
    OD: -175
    OS: -175

    OD CYL: spl
    OS CYL: spl

    Then in my axis and prism and etc. They're just blank, nothing written in the column. In the ADD column theres nothing written in them either. That's all the information I was given on my glasses prescription. -175 in both eyes.
    Can anyone help change it into contacts? I think it should be fairly easy...
    Hello asian and welcome to the forum.

    Your left and right eye spectacle prescription is -1.75 Dioptres. You don't have any reported astigmatism and you don't have presbyopia (you're probably still young!).

    The correct contact lens prescription is -1.71 Dioptres, but as this is not a standard contact lens size, you should select -1.75 Dioptre as this is the nearest value (and the same as your spectacle prescription).

    There are some questions over the health of circle lenses (at least in the US), so it would be advisable to have the circle lenses checked for fit on your eye by a qualified eye care practitioner. You don't say if you have worn contact lenses before, but if not, you should have your eyes checked to make sure they are compatible with circle contact lenses.

    knotlob

  63. #63

    Default

    Quote Originally Posted by Knotlob View Post
    Hello asian and welcome to the forum.

    Your left and right eye spectacle prescription is -1.75 Dioptres. You don't have any reported astigmatism and you don't have presbyopia (you're probably still young!).

    The correct contact lens prescription is -1.71 Dioptres, but as this is not a standard contact lens size, you should select -1.75 Dioptre as this is the nearest value (and the same as your spectacle prescription).

    There are some questions over the health of circle lenses (at least in the US), so it would be advisable to have the circle lenses checked for fit on your eye by a qualified eye care practitioner. You don't say if you have worn contact lenses before, but if not, you should have your eyes checked to make sure they are compatible with circle contact lenses.

    knotlob
    I understand. Are you sure its -1.71 or -1.75? I thought + was for farther distances and - for near sights in contacts. I thought it would be opposite of glasses...

  64. #64
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    Quote Originally Posted by asian View Post
    I understand. Are you sure its -1.71 or -1.75? I thought + was for farther distances and - for near sights in contacts. I thought it would be opposite of glasses...
    A + Dioptre lens is convex/fatter in the middle. A - Dioptre lens is concave/thinner in the middle. I can see things close, but not at all well a long distance away, so I wear glasses or contact lenses, which are concave, i.e. have a - dioptre power.

    If you cannot see things far away clearly without glasses/contact lenses, you will need a - value dioptre correction lens (spectacle or contact lens).

    (I also have presbyopia and therefore, difficulty focussing over a full range of close to far away - comes with age unfortunately. So if I correct my long distance vision for far away, driving, etc. then I would need a + correction reading spectacles to read small print, etc.)

    I hope that is clear.

    knotlob

  65. #65
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    Default Be Careful With Those Circle Lenses

    Quote Originally Posted by asian View Post
    I understand. Are you sure its -1.71 or -1.75? I thought + was for farther distances and - for near sights in contacts. I thought it would be opposite of glasses...
    Hello Asian.

    I can't help but wonder why you don't just get the prescription from your eye doctor. Sure that wouldn't make for very interesting forum reading, but I just hope you're not scribbling down those numbers and giving them to some illegal contact lens seller who does not require a prescription.

    Knotlob is right in that circle lenses can be risky. Be very careful when ordering them. Stay in close contact with your eye doctor in avoid trouble. Good luck.

  66. #66

    Default

    Quote Originally Posted by Knotlob View Post
    A + Dioptre lens is convex/fatter in the middle. A - Dioptre lens is concave/thinner in the middle. I can see things close, but not at all well a long distance away, so I wear glasses or contact lenses, which are concave, i.e. have a - dioptre power.

    If you cannot see things far away clearly without glasses/contact lenses, you will need a - value dioptre correction lens (spectacle or contact lens).

    (I also have presbyopia and therefore, difficulty focussing over a full range of close to far away - comes with age unfortunately. So if I correct my long distance vision for far away, driving, etc. then I would need a + correction reading spectacles to read small print, etc.)

    I hope that is clear.

    knotlob
    Thank you for your time. I would've thought the prescription would be knocked down a couple of degrees since the contacts will be sitting directly on my eyes.

  67. #67

    Default

    Quote Originally Posted by Scienceguy View Post
    Hello Asian.

    I can't help but wonder why you don't just get the prescription from your eye doctor. Sure that wouldn't make for very interesting forum reading, but I just hope you're not scribbling down those numbers and giving them to some illegal contact lens seller who does not require a prescription.

    Knotlob is right in that circle lenses can be risky. Be very careful when ordering them. Stay in close contact with your eye doctor in avoid trouble. Good luck.
    I'll contact my eye doctor for an exam very soon. Yes I understand circle lens have been labeled dangerous and I will be extremely careful when putting them in my eye. ^^ Thank you for your worries.

  68. #68
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    Default

    Quote Originally Posted by asian View Post
    Thank you for your time. I would've thought the prescription would be knocked down a couple of degrees since the contacts will be sitting directly on my eyes.
    Your prescription is only -1.75D so anything below about -4.00D is not very different between glasses and contact lenses.

    If you fancy the maths, look at the formula Sir Robin posted on this subject.

    knotlob

  69. #69

    Default

    Dear experts,

    my optician has prescribed to me the following

    R -675 -25*96
    L -725 -50*176

    Can I check if i'm going for contact lenses, what should be the correct power I will need???

  70. #70

    Default Not!

    Quote Originally Posted by Knotlob View Post
    Your prescription is only -1.75D so anything below about -4.00D is not very different between glasses and contact lenses.

    If you fancy the maths, look at the formula Sir Robin posted on this subject.

    knotlob
    Oh, I fancy the maths all right.

  71. #71
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    Quote Originally Posted by sorrow81 View Post
    Dear experts,

    my optician has prescribed to me the following

    R -675 -25*96
    L -725 -50*176

    Can I check if i'm going for contact lenses, what should be the correct power I will need???
    Hello sorrow81 and welcome to the forum.

    Your optician will apply a vertex correction formula to your spectacle prescription to arrive at a starting point for your contact lens strength. It will not necessarily be your optimum contact lens strength and the optician will determine that by trying over refraction lenses in front of your contact lenses and will of course check that the lenses float properly on your eye and that your eyes are healthy enough to wear contact lenses.

    So for your Right Eye:
    Spectacle prescription: R -6.75D (Spherical), -0.25D (Cylinder), 96 Deg (Axis)
    Contact prescription: R -6.24D (Spherical), -0.25D (Cylinder), 96 Deg (Axis). In practice the spherical correction will be rounded off to -6.25D. But assuming you don't have toric lenses, the optician could add half the astigmatism cylinder correction to your spherical figure (-6.24 -.125 = -6.37). So rounding off will probably still be -6.25D.

    Similarly for your Left eye:
    Spectacle prescript: L -7.25D (Spherical), -0.50D (Cylinder), 176 Deg (Axis)
    Contact prescription: L -6.67D (Spherical), -0.50D (Cylinder), 176 Deg (Axis).
    Spherical correction -6.67 (-half the astigmatism cylinder if no torics worn) -0.25D = -6.92, rounded off to -7.0D.

    Your right and left eye prescriptions for contacts would be :

    R -6.25 D (non toric lens)
    L -7.00 D (non toric lens)

    knotlob

  72. #72

    Default

    Vertex conversion and Rx adjustments are all well and good, but they are only part of the process. What are the oxygen requirements for the person ordering the lenses? How do you know what base curve and diameter to order? What are your corneal curvature measurements? What is your tear break-up time? What is your HVID? How do you know the lenses fit? If they are too loose, you'll know by the comfort and vision problems. However, if your lenses are too tight, the comfort and vision may well be quite good, but you'll cause permanent damage to your eyes without noticing any problem with the comfort or vision. For all the numbers and measurements and equations, the ONLY way to know if a lens fits is to have a proper visual inspection with a 'slit lamp" and keratometer.

  73. #73

    Wink

    Quote Originally Posted by Knotlob View Post
    Hello sorrow81 and welcome to the forum.

    Your optician will apply a vertex correction formula to your spectacle prescription to arrive at a starting point for your contact lens strength. It will not necessarily be your optimum contact lens strength and the optician will determine that by trying over refraction lenses in front of your contact lenses and will of course check that the lenses float properly on your eye and that your eyes are healthy enough to wear contact lenses.

    So for your Right Eye:
    Spectacle prescription: R -6.75D (Spherical), -0.25D (Cylinder), 96 Deg (Axis)
    Contact prescription: R -6.24D (Spherical), -0.25D (Cylinder), 96 Deg (Axis). In practice the spherical correction will be rounded off to -6.25D. But assuming you don't have toric lenses, the optician could add half the astigmatism cylinder correction to your spherical figure (-6.24 -.125 = -6.37). So rounding off will probably still be -6.25D.

    Similarly for your Left eye:
    Spectacle prescript: L -7.25D (Spherical), -0.50D (Cylinder), 176 Deg (Axis)
    Contact prescription: L -6.67D (Spherical), -0.50D (Cylinder), 176 Deg (Axis).
    Spherical correction -6.67 (-half the astigmatism cylinder if no torics worn) -0.25D = -6.92, rounded off to -7.0D.

    Your right and left eye prescriptions for contacts would be :

    R -6.25 D (non toric lens)
    L -7.00 D (non toric lens)

    knotlob
    Hi Knotlob, thanks for the welcome. I have actually tried to apply Sir Robin's formula found in this long thread but I can't seem to get the calculations correct, how did u manage to get the calculations?

    I have been trying it for the past 1 hr yesterday, maybe i'm poor in maths, thus I can't get the proper calculations. Would appreciate if you can enlighten me on this matter ya thanks a million

  74. #74

    Default

    Quote Originally Posted by sorrow81 View Post
    Hi Knotlob, thanks for the welcome. I have actually tried to apply Sir Robin's formula found in this long thread but I can't seem to get the calculations correct, how did u manage to get the calculations?

    I have been trying it for the past 1 hr yesterday, maybe i'm poor in maths, thus I can't get the proper calculations. Would appreciate if you can enlighten me on this matter ya thanks a million
    I think i managed to get it though the figures vary very slightly this webby is really informative manz

  75. #75
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    Default

    Quote Originally Posted by sorrow81 View Post
    I think i managed to get it though the figures vary very slightly this webby is really informative manz
    Hi sorrow81

    I actually put the formula into a spreadsheet some time ago and took some time to make sure it is accurate. But you need to be very careful with brackets, etc. so that you only multiply/divide the bits of the formula that you should be.

    knotlob

  76. #76

    Default

    Quote Originally Posted by Knotlob View Post
    Hi sorrow81

    I actually put the formula into a spreadsheet some time ago and took some time to make sure it is accurate. But you need to be very careful with brackets, etc. so that you only multiply/divide the bits of the formula that you should be.

    knotlob
    ahh ic ic... thanks for the help, really appreciate it... u guys must be pros knowing such terms...

  77. #77

    Default Smart People

    Quote Originally Posted by sorrow81 View Post
    ahh ic ic... thanks for the help, really appreciate it... u guys must be pros knowing such terms...
    Hi sorrow81. There are a lot of really smart people on this forum.

    And then there's me.

  78. #78

    Default

    Wow this site is amazing. People are so knowledgable!!

    I don't know if I am using the two sites correctly. http://204.8.122.98:8888/EXEC/0/1e4jthr0ph50la13yqvdy0aitimr and http://virtualconsultant.cibavision.com/best_match.jsp

    I find the Cibavision website to be WAY easier to figure out. But when I completed both of the forms on each site I got a different contact lense prescription for each website.

    On the CIBAvision it came back with: -1.25/-1.75 x 180 for my right eye and -1.75/-1.25 x 10.

    The other site it was -1.00/-2.00 x 180 for my right eye and -1.75/-1.50 x 10.

    Can anyone help me? My eyeglass prescription is:

    OD -1.25/-2.00 x 180
    OS -1.75/-1.50 x 012

    Thanks!!

  79. #79
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    Default

    Quote Originally Posted by ChelseaLynn View Post
    Wow this site is amazing. People are so knowledgable!!

    I don't know if I am using the two sites correctly. http://204.8.122.98:8888/EXEC/0/1e4jthr0ph50la13yqvdy0aitimr and http://virtualconsultant.cibavision.com/best_match.jsp

    I find the Cibavision website to be WAY easier to figure out. But when I completed both of the forms on each site I got a different contact lense prescription for each website.

    On the CIBAvision it came back with: -1.25/-1.75 x 180 for my right eye and -1.75/-1.25 x 10.

    The other site it was -1.00/-2.00 x 180 for my right eye and -1.75/-1.50 x 10.

    Can anyone help me? My eyeglass prescription is:

    OD -1.25/-2.00 x 180
    OS -1.75/-1.50 x 012

    Thanks!!
    The formula that Sir Robin posted gives:

    Right Eye Contact Lens: -1.23D, -2.00D, 180 Deg
    Left Eye Contact Lens: -1.71D, -1.50D, 012 Deg

    Sir Robin's formula only corrects for spherical prescriptions.

    The cylinder axis and astigmatism prescription values (2nd and 3rd figures in your prescription) need to be rounded off to the nearest available in your chosen contact lens.

    Similarly the spherical prescriptions would need to be rounded up or down to suit the contact lens availability.

    I am assuming you will be wearing toric lenses, so there is no half astigmatism correction applied to the spherical correction in these cases.

    As you can see there is not much vertex correction at low spherical prescriptions. The vertex correction becomes more pronounced at greater than -4.00D spherical prescription.

    knotlob

  80. #80

    Default

    Quote Originally Posted by KSCbase View Post
    I couldn't get it to work either. I have my prescription with me, so I clicked "Disposable/Frequent Replacement" and then on the "Toric" button, copied the sphere, cylinder and axis numbers from my prescription for both eyes, put in a vertex distance of 12, no "add" power because there was none in my prescription, clicked "Recommend" and all I got for an answer was a Contact Name, Contact Number, and Email. No brand recommendations. What did I do wrong?
    It seems like that virtualconsultant.cibavision.com website recommended by jpeg4 is not very user-friendly when it comes to mere mortals. Does anyone have a simpler contact lens prescription calculator web site . . . thingee?
    Something where people like me can use the glasses prescription card the they got from their eye doctor's office on their last visit and just plug in the sphere, cylinder and if necessary, the axis numbers from the card, listen to the website chug and watch the lights flash, and have it spit out an equivalent contact lens prescription and possibly even some brand recommendations?
    Attached Images Attached Images

  81. #81

    Default

    Hi Chelsee,

    I had a hard time figuring out how to use the CIBAvision website but when you first pull up the website you need to make sure you go to "Product Match". Then choose disposable or conventional contacts. Vertex distance is usually 12 for most people (vertex distance is the distance from your glasses to the front of your cornea). Then in OD right put in your right eye prescription information. Below that it says add power. That can be 0 if it is not on your prescription. THen choose toric (for astigmatism) or sphere. Do the same for your left eye and hit calculate and it will show your prescription and also recommend you a few brands of contact lenses.

    I hope this helps because I had a hard time too until I realized I wasn't on the right page.

  82. #82

    Default Useless Form

    Quote Originally Posted by ChelseaLynn View Post
    Hi Chelsee,

    I had a hard time figuring out how to use the CIBAvision website but when you first pull up the website you need to make sure you go to "Product Match". Then choose disposable or conventional contacts. Vertex distance is usually 12 for most people (vertex distance is the distance from your glasses to the front of your cornea). Then in OD right put in your right eye prescription information. Below that it says add power. That can be 0 if it is not on your prescription. THen choose toric (for astigmatism) or sphere. Do the same for your left eye and hit calculate and it will show your prescription and also recommend you a few brands of contact lenses.

    I hope this helps because I had a hard time too until I realized I wasn't on the right page.
    Woah. Long thread.

    I tried that website at virtualconsultant.cibavision.com, and when I plugged in the numbers for my glasses prescription, I didn't get any kind of "try this brand of contact lens" results. Only the "Professional Consultation" message that Eyeofthetiger said he got. Maybe it just doesn't like people who use "tiger" in their screen names on Lens 101?

    Just below the soothing aqua-colored form it said:
    Contact Name : Professional Consultation
    Contact Number : 800-241-7468
    Email : tccr.atlanta@cibavision.com

    That stuff is there even before you start entering numbers, so the form did nothing.

    Zis form! It does nothing!
    Attached Images Attached Images

  83. #83

    Default

    Quote Originally Posted by Tradecade View Post
    Hi sorrow81. There are a lot of really smart people on this forum.

    And then there's me.
    haha that i definitely believe

  84. #84

    Default Advice for 11year old son with minor perscription who wants contacts

    Hi,
    I am brand new to this site, but have read all the posts on converting a glasses perscription into a contact lens perscription.

    Background information: My son, who is 11 years old, and just started playing football, came home yesterday requesting we look into getting contact lens for him since glasses are going to be hard in the football helmet. Unfortunately, our vision insurance only covers once a year eye exams and either contacts or glasses. He had his last eye exam January, 2010. So, I called his eye doc. to see what he would charge us for the contact lens perscrption and exam and it is going to be $150 out of pocket ~ UGH!

    So, I am now tempted to just order some online for my son until we can have our insurance pay for the exam which will be in about 5 months from now.

    Having read the posts it seems that his correction is pretty minor and therefore wouldn't too big of an issue to convert his eye glass perscription to the contact lens perscription. However, can someone let me know if this is the case? Also, what would his conversion be?

    Eye glass perscription:
    OD -1.00 -.50 x 090
    OS -1.00 -0.25 x 067

    I realize it is optimal to take him in to the eye doc. for the contact lens exam, and I PROMISE I will in 5 months from now when our insurance will pay the $150.

    Thanks for your help!

  85. #85
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    Default

    Quote Originally Posted by momma_blue_eyes View Post
    Hi,
    I am brand new to this site, but have read all the posts on converting a glasses perscription into a contact lens perscription.

    Background information: My son, who is 11 years old, and just started playing football, came home yesterday requesting we look into getting contact lens for him since glasses are going to be hard in the football helmet. Unfortunately, our vision insurance only covers once a year eye exams and either contacts or glasses. He had his last eye exam January, 2010. So, I called his eye doc. to see what he would charge us for the contact lens perscrption and exam and it is going to be $150 out of pocket ~ UGH!

    So, I am now tempted to just order some online for my son until we can have our insurance pay for the exam which will be in about 5 months from now.

    Having read the posts it seems that his correction is pretty minor and therefore wouldn't too big of an issue to convert his eye glass perscription to the contact lens perscription. However, can someone let me know if this is the case? Also, what would his conversion be?

    Eye glass perscription:
    OD -1.00 -.50 x 090
    OS -1.00 -0.25 x 067

    I realize it is optimal to take him in to the eye doc. for the contact lens exam, and I PROMISE I will in 5 months from now when our insurance will pay the $150.

    Thanks for your help!
    Hi momma_blue_eyes and welcome to the forum.

    I understand what you are saying about the cost of the eye exam, but it is critical that you have your son's eyes examined specifically for contact lenses. It is just not as simple as converting his spectacle prescription to a contact lens prescription. His eyes MUST be examined to ensure that they are healthy and suitable for contact lenses. If you don't do this and buy lenses from some Mail Order company that don't fit properly, you could do permanent damage to your son's eyes.

    When your son is fitted for contacts in January, the eye care professional should also explain about the different types of contact lenses, how to look after them, cleaning/disinfection, storing them, not sleeping in them, how long to wear them, etc. Also what to do if things go wrong and he gets an adverse reaction to them or an infection in the eye.

    It simply isn't worth the risk. It is now September so I would wait until the eye exam in January before you try the contacts. Your son will be disappointed for sure, but it is for his own good.

    As you said, the prescription is very minor and the optician would not correct the astigmatism with contacts as this too is so minor. Your optician will probably start out with a similar prescription as your son's spectacles for the OS (Left lens) with perhaps a -1.25D correction in the OD (right lens). But that has to be fine tuned when the contact lenses are in the eye and the eye sight is checked against the eyesight card.

    Another thing to consider is that your son's eyes will be changing very rapidly over the next few years as he goes through a growth spurt, so the fit and strength of the contact lens needs to be checked regularly (annual eye exam). My son got contact lenses at 13 years. These were Rigid Gas Permeable lenses, which the optician believed may slow down the rate of change in my son's eyesight. RGP lenses do however, need very careful fitting and you cannot just pick some lenses out of a Mail Order catalogue.

    Soft lenses are easier, possibly Daily Disposables, but I strongly recommend that you let the eye care practitioner fit the contact lenses properly. Once you have a professionally determined prescription, you can buy soft lenses from a Mail order company and potentially save money, but you should ensure that your son has an annual eye exam with the contact lenses.

    knotlob

  86. #86

    Default

    I got back from the doctor with my new eye glasses. I was wondering how you eould calculate my prescription from my left eye -1.75 and my right eye -1.50. I'm not a math wiz so I can't calculate it on my own. :/

  87. #87

    Default

    I got back from the doctor with my new eye glasses. I was wondering how you eould calculate my prescription from my left eye -1.75 and my right eye -1.50. I'm not a math wiz so I can't calculate it on my own. :/

  88. #88
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    Default

    Quote Originally Posted by Sal18 View Post
    I got back from the doctor with my new eye glasses. I was wondering how you eould calculate my prescription from my left eye -1.75 and my right eye -1.50. I'm not a math wiz so I can't calculate it on my own. :/
    Hello Sal18 and welcome to the forum.

    Why do you want to calculate the contact lens prescription?

    In your case, the spectacle prescription is relatively low, so there is no practical difference between the spectacle and contact lens prescription.

    But it is not as simple as buying a pair of contact lenses from an on-line store or a vending machine or WalMart, etc.

    You must have your Eye Care Professional examine your eyes specifically for contact lens wear (not spectacle wear) and then if your eyes are OK, then your Eye Care Professional will be able to recommend and possibly trial you with free lenses to see how they fit. They would also explain how long you can wear the lenses, how to put them in, take them out, disinfect them, etc. What to do when something goes wrong, etc. WalMart, etc. don't do that unless they have an Eye Care Specialist on the premises who can examine your eyes.

    Once you have got a prescription (which you will need anyway to buy contact lenses in the USA) you can buy from a Mail Order store, probably significantly cheaper than from your Eye Care Specialist. You need to have an eye exam once/year to check that your eyes are still OK with the contact lenses.

    knotlob

  89. #89

    Default

    Hi I was hoping some on could help me out.My eyeglass prescription is as follows;

    Right eye: -2.50sph x -0.75cyl x 135.0 axis

    Left eye: -1.50sph x -1.50cyl x 65.0 axis

    Now here's the problem I used the calculator and got back;

    Right eye: -2.50sph x -0.75cyl x 140.0 axis

    Left eye: -1.50sph x -1.25cyl x 70.0 axis

    I know I have a astigmatism in one eye,though I don't know which one,but am I suppost to use astigmatism contacts for both? If i choose any other non-astigmatism contact;

    Right eye: -2.75

    Left eye: -2.25
    That just seems like too big of jump to me. Please help! I just got the prescription on 9/8/10 and they want $133.00 fitting fee for contacts. Thank you
    Sloegin354

  90. #90

    Default

    Hi I was hoping some on could help me out.My eyeglass prescription is as follows;

    Right eye: -2.50sph x -0.75cyl x 135.0 axis

    Left eye: -1.50sph x -1.50cyl x 65.0 axis

    Now here's the problem I used the calculator and got back;

    Right eye: -2.50sph x -0.75cyl x 140.0 axis

    Left eye: -1.50sph x -1.25cyl x 70.0 axis

    I know I have a astigmatism in one eye,though I don't know which one,but am I suppost to use astigmatism contacts for both? If i choose any other non-astigmatism contact;

    Right eye: -2.75

    Left eye: -2.25
    That just seems like too big of jump to me. Please help! I just got the prescription on 9/8/10 and they want $133.00 fitting fee for contacts. Thank you
    Sloegin354

  91. #91
    Join Date
    Nov 2009
    Location
    near Hamburg, Germany
    Posts
    2,144

    Default

    Quote Originally Posted by sloegin354 View Post
    Hi I was hoping some on could help me out.My eyeglass prescription is as follows;

    Right eye: -2.50sph x -0.75cyl x 135.0 axis

    Left eye: -1.50sph x -1.50cyl x 65.0 axis

    Now here's the problem I used the calculator and got back;

    Right eye: -2.50sph x -0.75cyl x 140.0 axis

    Left eye: -1.50sph x -1.25cyl x 70.0 axis

    I know I have a astigmatism in one eye,though I don't know which one,but am I suppost to use astigmatism contacts for both? If i choose any other non-astigmatism contact;

    Right eye: -2.75

    Left eye: -2.25
    That just seems like too big of jump to me. Please help! I just got the prescription on 9/8/10 and they want $133.00 fitting fee for contacts. Thank you
    Sloegin354
    Hello Sloegin354 and welcome to the forum.

    A lot of people come onto the forum and ask for a contact lens prescription based on a spectacle prescription. I realise you may be charged extra for a contact lens examination, but surely you can get a reduced price if you have just been fitted with spectacles (or is the $133 a reduced price)?. It is your eyes we are talking about and if you damage them with badly fitting contacts, then I think medical costs to return your eyes to normal (if that is indeed possible) will greatly exceed the cost of the second eye examination for contacts.

    I have cut and paste the warning I wrote for the previous poster:

    It is not as simple as buying a pair of contact lenses from an on-line store or a vending machine or WalMart, etc.

    You must have your Eye Care Professional examine your eyes specifically for contact lens wear (not spectacle wear) and then if your eyes are OK, then your Eye Care Professional will be able to recommend and possibly trial you with free lenses to see how they fit. They would also explain how long you can wear the lenses, how to put them in, take them out, disinfect them, etc. What to do when something goes wrong, etc. WalMart, etc. don't do that unless they have an Eye Care Specialist on the premises who can examine your eyes.

    Once you have got a prescription (which you will need anyway to buy contact lenses in the USA) you can buy from a Mail Order store, probably significantly cheaper than from your Eye Care Specialist. You need to have an eye exam once/year to check that your eyes are still OK with the contact lenses.

    In your case Sloegin354, you appear to have used some on-line calculator.

    eyeglass prescription is as follows;
    Right eye: -2.50sph x -0.75cyl x 135.0 axis
    Left eye: -1.50sph x -1.50cyl x 65.0 axis

    You have astigmatism in BOTH eyes, though your right eye (cylinder) is only -0.75D, so this is not normally corrected by eye care specialists. Your left eye has more astigmatism, so it would normally be corrected using a toric lens.

    The on-line calculator is changing some values (e.g. the axis and cylinder) in order to match contact lenses available from whoever provided the on-line calculator.

    The figures look correct. The first set of values is correction using toric lenses in both eyes (-0.75D cylinder is the minimum correction normally available, but not in all brands of contact lenses).

    The second set of figures are based on spherical (non toric) correction lenses only. The reason the strengths are higher (more negative) is that the calculator has taken half the astigmatism (cylinder) value and added it to the spherical correction. This is standard practice if you don't correct the astigmatism with toric lenses.

    You would probably get better vision with properly fitted toric lenses, though they tend to be more expensive and can be awkward to fit to ensure that the lenses don't rotate off their specified axis for the astigmatism.

    It is better if you let your eye care specialist fit you with the contact lenses, particularly in view of your astigmatism. You will need a prescription anyway in order to buy contact lenses on-line, assuming you are in the USA.

    knotlob

  92. #92

    Question Holy toledo batman*****

    This is ALL SO Confusing....
    I was wondering, since I can see you like, and are Very capable of converting RX's,, here's one for ya!!
    I do have a current RX,, and my eye Dr, is mailing it to me,, I just have NO patience....
    And they would not give me my RX over the phone??
    But, here is my last eyeglasses exam...
    Could you possibly be so kind, and convert it into a contact RX... : )
    I know, I have astigmatism,, and I kow, not all colored contacts can fix that problem, but,, I wanna see what colors are out there for me!!
    Thanks bunches.
    : )

    Darnit,,
    I almost forgot...
    O.D SPH+100, CYL-125, axis 179
    O.S SPH+025, CYL-100, axis176

  93. #93
    Join Date
    Nov 2009
    Location
    near Hamburg, Germany
    Posts
    2,144

    Default

    Quote Originally Posted by sproutjps View Post
    This is ALL SO Confusing....
    I was wondering, since I can see you like, and are Very capable of converting RX's,, here's one for ya!!
    I do have a current RX,, and my eye Dr, is mailing it to me,, I just have NO patience....
    And they would not give me my RX over the phone??
    But, here is my last eyeglasses exam...
    Could you possibly be so kind, and convert it into a contact RX... : )
    I know, I have astigmatism,, and I kow, not all colored contacts can fix that problem, but,, I wanna see what colors are out there for me!!
    Thanks bunches.
    : )

    Darnit,,
    I almost forgot...
    O.D SPH+100, CYL-125, axis 179
    O.S SPH+025, CYL-100, axis176
    Hello Sproutjps

    OK, I can give you the contacts prescription, but it will not be very different to the spectacle prescription, since your spherical (i.e. non astigmatism) correction is small. The prescription tends to change more, when the spherical prescription is stronger than -4.00D.

    So, your Spectacle prescription is:
    O.D (Right Eye) +1.00D (Sph), -1.25D (Cyl), 179 Deg (axis)
    O.S (Left Eye) +0.25D (Sph), -1.00D (Cyl), 176 Deg (axis)
    You are slightly long sighted.

    Using the Vertex Correction (spherical) converter for your eyes, Contact Lens prescription is:
    O.D (Right Eye) +1.01D (Sph), -1.25D (Cyl), 179 Deg (axis)
    O.S (Left Eye) +0.25D (Sph), -1.00D (Cyl), 176 Deg (axis)

    i.e. no change here, because of the low spherical correction. These values are what you would use as a starting point for the toric contact lenses. You would round down the Right Eye Spherical correction from +1.01D to +1.00D and you would have to round off the axis values to say 180 Deg, or whatever is nearest available in contact lenses.

    The spherical value would change a little if you did not correct for astigmatism, but since you are already wearing non-toric contacts, you know what that prescription is anyway I think.

    You can use these values to see what coloured lenses are available and as usual I recommend that you do let the Eye Care Specialist do the toric lens fitting, as it sometimes needs to be adjusted. You have such attractive eyes, I wouldn't like anything to damage them through badly fitted contact lenses .

    knotlob

  94. #94

    Default Super!!! Thanks

    Wow, you are AWESOME!!
    Thanks a bunch!
    I love being on this lense forum, I have gained so much knowledge!
    : )

  95. #95
    Join Date
    Dec 2007
    Location
    United States of America
    Posts
    274

    Default

    Quote Originally Posted by sorrow81 View Post
    haha that i definitely believe
    Hey, wait a minute. Did you just insult Tradecade?

  96. #96

    Default

    Hasn't anyone ever heard of 'power gain due to thickness' when adjusting cylinder for contact lenses.

  97. #97
    Join Date
    Nov 2009
    Location
    near Hamburg, Germany
    Posts
    2,144

    Default

    Quote Originally Posted by Contact Lens Fitter View Post
    Hasn't anyone ever heard of 'power gain due to thickness' when adjusting cylinder for contact lenses.
    Is that the addition of half the cylinder value to the spherical value if torics are not prescribed, or another correction? Your knowledge is always welcomed.

    knotlob

  98. #98

    Default

    Adding half the cylinder to the sphere only works in some cases and depends on the thickness and stiffness of the specific lens being used. Power gain due to thickness is an increase in the strength of the Rx along the cylinders axis due to the extra thickness. As a rule, the cylinder increase is aprox. 30%. I have seen patients with a cyl of -4.00 be corrected to 20/20 with a -2.75 cyl correction. When it comes to correcting astigmatism, less is more. Don't forget, if the Rx is -6.50-3.00x180, the power along 180 is -6.50 but the power along 90 is NOT -3.00, it's -9.50 !!! Add to that the 'power gain' and we find out why some people say torics don't give them a good correction. Many people are over-corrected.

  99. #99
    Join Date
    Nov 2009
    Location
    near Hamburg, Germany
    Posts
    2,144

    Default

    Quote Originally Posted by Contact Lens Fitter View Post
    Adding half the cylinder to the sphere only works in some cases and depends on the thickness and stiffness of the specific lens being used. Power gain due to thickness is an increase in the strength of the Rx along the cylinders axis due to the extra thickness. As a rule, the cylinder increase is aprox. 30%. I have seen patients with a cyl of -4.00 be corrected to 20/20 with a -2.75 cyl correction. When it comes to correcting astigmatism, less is more. Don't forget, if the Rx is -6.50-3.00x180, the power along 180 is -6.50 but the power along 90 is NOT -3.00, it's -9.50 !!! Add to that the 'power gain' and we find out why some people say torics don't give them a good correction. Many people are over-corrected.
    Thanks very much. I understood that two lenses in one (spherical + toric) complicated the prescription, but as you explain it, the power increase will apply on the cylinder axis and not universally over the whole lens.

    I will PM you early next week to see if I can fully understand the correction calculation, but it sounds like it is only an approximation (as I had always believed) and the actual correct/optimum prescription must be found by actual trial and error by the contact lens fitter.

    Thanks

    knotlob

  100. #100

    Default

    If it was safe and legal to dispense contact lenses by just converting a spectacle RX to a contact lens RX people wouldn't have to post their RX on the web for help. The internet vendors would be doing the conversion for you.

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