How Do Bifocal Contact Lenses Work?

Unicorn3 · #1 (**permalink**) 01-30-2008, 04:19 PM

I'm having a hard time visualizing how bifocal contact lenses work. With bifocal eyeglasses the lenses are bigger than your eyes, so when you look down at something you're reading you're looking through the bottom half of the lenses.
With Bifocal Contact Lenses don't the lenses move with your eyes so that you're always looking through the center? How would a bifocal lens work if that's the case?

Droopy · #2 (**permalink**) 11-22-2011, 10:04 AM

Quote:

Originally Posted by Unicorn3

I'm having a hard time visualizing how bifocal contact lenses work. With bifocal eyeglasses the lenses are bigger than your eyes, so when you look down at something you're reading you're looking through the bottom half of the lenses.
With bifocal contact lenses don't the lenses move with your eyes so that you're always looking through the center? How would a bifocal lens work if that's the case?

As I understand it, when you look down, the lenses shift a little on your eye, allowing you to see through the bottom part.

There are other lenses when the near vision part is in the center of the lenses and the distance part is on the edges. Somehow it works out when you're wearing them, but I'm not sure how.

If anyone else can explain it better, please feel free.

Timbo59 · #3 (**permalink**) 11-22-2011, 01:45 PM

There are two main ways of doing this:
1) a small wedge at the bottom of the lens in much the same way as bi-focal spectacles. They will need some way of keeping the bottom bit at the bottom, some rely just on the extra mass, some have a flattened base to the lens sits on the lower lid. As you say, you then look down, and the eyelid keeps the lens in place and you end up looking through the wedge at the bottom of the contact lens. I believe this option is used mainly, if not totally for RGP lenses.

2) Concentric zones. These have a distance prescription in the centre of the lens, and a near prescription in the outer zone (or the other way around depending on what you want to use them for - you can get both sorts).

In these, you look ahead and get sharp distance vision through the centre section. As this area is a little small, you will also get some image that goes through the near-zone at the edge. This means you get a sharp image, and to some extent a fuzzy image mixed in with it. It looks a bit like a mild soft-focus filter on a photograph, after a while you hardly notice it.
When you look down, you tend to look through more of the near-zone, and so the close things will be sharper. These lenses would be good for someone in an office, say. I'm trying out some RGPs like this at the moment.

If you turn the zones around, so near vision is in the centre and distance around the outside, they might be better for a sportsman, who would need sharper distance peripheral vision.

Englishteacher1 · #4 (**permalink**) 11-22-2011, 02:25 PM

Quote:

Originally Posted by Timbo59

There are two main ways of doing this:
1) a small wedge at the bottom of the lens in much the same way as bi-focal spectacles. They will need some way of keeping the bottom bit at the bottom, some rely just on the extra mass, some have a flattened base to the lens sits on the lower lid. As you say, you then look down, and the eyelid keeps the lens in place and you end up looking through the wedge at the bottom of the contact lens. I believe this option is used mainly, if not totally for RGP lenses.

2) Concentric zones. These have a distance prescription in the centre of the lens, and a near prescription in the outer zone (or the other way around depending on what you want to use them for - you can get both sorts).

In these, you look ahead and get sharp distance vision through the centre section. As this area is a little small, you will also get some image that goes through the near-zone at the edge. This means you get a sharp image, and to some extent a fuzzy image mixed in with it. It looks a bit like a mild soft-focus filter on a photograph, after a while you hardly notice it.
When you look down, you tend to look through more of the near-zone, and so the close things will be sharper. These lenses would be good for someone in an office, say. I'm trying out some RGPs like this at the moment.

If you turn the zones around, so near vision is in the centre and distance around the outside, they might be better for a sportsman, who would need sharper distance peripheral vision.

Thanks, Timbo59. That kind of thing is hard to explain without pictures, and I think you did a good job. Well done.

PrincePlanet · #5 (**permalink**) 01-10-2012, 09:39 AM

Quote:

Originally Posted by Timbo59

There are two main ways of doing this:
1) a small wedge at the bottom of the lens in much the same way as bi-focal spectacles. They will need some way of keeping the bottom bit at the bottom, some rely just on the extra mass, some have a flattened base to the lens sits on the lower lid. As you say, you then look down, and the eyelid keeps the lens in place and you end up looking through the wedge at the bottom of the contact lens. I believe this option is used mainly, if not totally for RGP lenses.

2) Concentric zones. These have a distance prescription in the centre of the lens, and a near prescription in the outer zone (or the other way around depending on what you want to use them for - you can get both sorts).

In these, you look ahead and get sharp distance vision through the centre section. As this area is a little small, you will also get some image that goes through the near-zone at the edge. This means you get a sharp image, and to some extent a fuzzy image mixed in with it. It looks a bit like a mild soft-focus filter on a photograph, after a while you hardly notice it.
When you look down, you tend to look through more of the near-zone, and so the close things will be sharper. These lenses would be good for someone in an office, say. I'm trying out some RGPs like this at the moment.

If you turn the zones around, so near vision is in the centre and distance around the outside, they might be better for a sportsman, who would need sharper distance peripheral vision.

With those concentric ring contact lenses, how do you not look through the center of the lens?

Droopy · #6 (**permalink**) 01-18-2012, 05:06 PM

Quote:

Originally Posted by PrincePlanet

With those concentric ring contact lenses, how do you not look through the center of the lens?

I guess like many things in life, it's a matter of focus.

Timbo59 · #7 (**permalink**) 01-19-2012, 03:16 AM

You will always look trough the centre of the lens, but depending on where you are looking - in RGP lenses at least - the lens isn't always in the same place on the eye you may be looking through more or less of the centre section.

Garfield21 · #8 (**permalink**) 01-19-2012, 09:55 AM

Quote:

Originally Posted by Timbo59

You will always look trough the centre of the lens, but depending on where you are looking - in RGP lenses at least - the lens isn't always in the same place on the eye you may be looking through more or less of the centre section.

Oh, so the contact lenses move a little in your eyes?