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View Full Version : Is a contact lens actually a lens?


tofergregg
01-17-2006, 10:53 PM
As a physics teacher, I should probably know this. However, I am still confused by how, exactly, a contact lens gives you better vision. My understanding is that it is not an actual lens in the sense that a glass (or other medium) lens refracts light because of the curved shape and refractive index of the material. Does the contact bend your cornea a bit, squeezing your eye to the correct shape? I'm curious. Thanks!

-Tofer

Q.E.D.
01-17-2006, 11:04 PM
No, the lens doesn't distort the corneal surface to any significant degree. Contact lenses work the same way that eyeglasses do, only the design of the curvature is more complex. Not only does the lens need to be ground and polished to the correct curvature for proper vision correction, but the inner surface must also be designed to accomodate the unique curvature of the wearer's cornea. It has to hold the lens in the right position, and in the case of astigmatism correction, at the right orientation, as well. A lot of variables have to be factored in for a correct and comfortable fit. But, the general principle behind them is still the same as any lens.

gotpasswords
01-18-2006, 11:28 AM
No, the lens doesn't distort the corneal surface to any significant degree.
Just wanted to mention that this is true for normal day-to-day life, but as always, there is a special case. If you're contemplating LASIK, you need to go without contacts for two weeks as daily use of contacts does alter the shape of the cornea very slightly.

You could go from long-term daily wear of contact lenses to LASIK on the same day, but it just makes sense to let the cornea be at its natural shape before embarking on an expensive procedure that is intended to permanently alter the shape of it.

CalMeacham
01-18-2006, 11:35 AM
Having built equipment to measure the optical powers of contact lenses, I can assure you that they are lenses in the normal sense. They're made of hydropolymers that incorporate a lot of water, and their indices of refraction are lower than those of most plastics (but try getting the manufacturers to tell you whast that value is -- it's worse than the Lite Beer manufacturers Cecil talked about), but they are ordinary lenses. We built one device that measured their properties in saline solution (where the lenses are unfolded and in their normal shape -- in air they stick together when wet and distort under gravity) and extrapolated back to their power in air.

"Hard" contact lenses are PMMA or some other plastic, and hold their shape and power even in the air. Pepper Mill uses these, and if you lookm through them you'll see that they act like regular lenses.


I've got a book on this -- Contact Lens Optics and Lens Design by Douthwaite. Oddly, ophthalmologists seem to speak a different language of optics than physicists and optical engineers.

FlyingStart
01-18-2006, 02:22 PM
QED isn't quite correct in his statement. Some time ago contacts were available on the high street that were made to match the curvature of the wearer's eye. A series of concentric rings were reflected off the eye and photographed to form a relief map for the manufacturers. This was for hard contacts only, simply to give a more comfortable fit and the practice was phased out about 1986 because it was too expensive.

Soft contact lenses follow the contours of the eye very exactly and as such cannot be prescribed for people with astigmatism. Toric lenses (soft lenses with a different central compound) are available to address this to a limited extent.

Rigid (hard or gas permeable) lenses have constant curvature. It is the fact that they are worn in direct contact with the eye, with a film of tears filling in the gaps, that makes them correct astigmatism. Astigmatism gives a "rugby ball" shape to the eye, the hard contact lens corrects it back to a "soccer ball" globe shape.

Cheers.

Feydeau
01-18-2006, 02:47 PM
QED isn't quite correct in his statement. Some time ago contacts were available on the high street that were made to match the curvature of the wearer's eye. A series of concentric rings were reflected off the eye and photographed to form a relief map for the manufacturers. This was for hard contacts only, simply to give a more comfortable fit and the practice was phased out about 1986 because it was too expensive.

Soft contact lenses follow the contours of the eye very exactly and as such cannot be prescribed for people with astigmatism. Toric lenses (soft lenses with a different central compound) are available to address this to a limited extent.

Rigid (hard or gas permeable) lenses have constant curvature. It is the fact that they are worn in direct contact with the eye, with a film of tears filling in the gaps, that makes them correct astigmatism. Astigmatism gives a "rugby ball" shape to the eye, the hard contact lens corrects it back to a "soccer ball" globe shape.

Cheers.

I wear one toric lens, one non-toric. (I'm not an eye professional of any kind, just an end-user.) I wear the extended-use kind; I put them in for a week (or slightly longer if they're still comfortable) then take them out and soak them in saline for a day or two, then put them back in for as long as they're comfortable (normally 3 to 8 days) then discard them.

My understanding of toric lens-shape is that the lens itself is thicker along one edge, causing that arc of the lens to always be on the bottom (as looking directly into the eye). If toric lenses have some different component in the center, I'm not aware of it. Of course, it could be that they didn't tell me as it's one of those "you won't understand" types of things.

So -- are there any contact lens pros out there that can tell us about the construction of toric lenses?

Q.E.D.
01-18-2006, 02:59 PM
Rigid (hard or gas permeable) lenses have constant curvature. It is the fact that they are worn in direct contact with the eye, with a film of tears filling in the gaps, that makes them correct astigmatism.
Sorry, but this simply is incorrect. In order to correct vision with astigmatism, the lens must have both a spherical component (the "constant curvature" you refer to) and a cylindrical component. In order for the astigmatism correction to perform properly, the cylindrical axis of the lens must be aligned correctly with respect to the astigmatism. There are a few ways to do this. If the astigmatism is severe enough--that is, if the cylindrical component is large--the inner curve of the lens can be shaped so that it naturally "floats" on the cornea in the correct orientation. For less severe astigmatism, the lens can be weighted towards one edge so that it rotates into the correct orientation. The latter will only work when your head is upright (or uside-down); if you tilt your head, the lens will rotate out of alignment and things will get blurry.

WhyNot
01-18-2006, 03:18 PM
Sorry, but this simply is incorrect. In order to correct vision with astigmatism, the lens must have both a spherical component (the "constant curvature" you refer to) and a cylindrical component. In order for the astigmatism correction to perform properly, the cylindrical axis of the lens must be aligned correctly with respect to the astigmatism. There are a few ways to do this. If the astigmatism is severe enough--that is, if the cylindrical component is large--the inner curve of the lens can be shaped so that it naturally "floats" on the cornea in the correct orientation. For less severe astigmatism, the lens can be weighted towards one edge so that it rotates into the correct orientation. The latter will only work when your head is upright (or uside-down); if you tilt your head, the lens will rotate out of alignment and things will get blurry.
(bolding mine)

...but it doesn't. I'm a RGP lens wearer with a severe astigmatism, and my vision upside down with my contacts in is fine. Their is no weighting to my lenses. Once, I had a lens with a small black dot on it (so I could tell left from right if I got them mixed up.) Sometimes the dot was on top. sometimes to the side - no consistent placement which would indicate an "up" or "down" to the lens.

CookingWithGas
01-18-2006, 03:21 PM
I have no idea which statements are correct but I wanna see a cite.

Q.E.D.
01-18-2006, 03:46 PM
Sorry, but this simply is incorrect.
I should modify that a bit--it's not entirely incorrect. Some very mild corneal astigmatism can, indeed, be corrected with a spherical lens which uses the film of tear fluid to form part of the refractive mechanism. But, this is mostly useless for more severe corneal astigmatism and is completely ineffective for lenticular astigmatism (where the lens of the eye, rather than the cornea, is cylindrically deformed). For the latter, the lens must designed to orient itself properly with respect to the eye irrespective of the corneal contour. One way to do this is by weighting--admittedly, I don't know how common this is anymore, but it's the first technique I remember hearing of. Another is to shape the lens with varying edge thickness, so that the normal blinking of the eyes causes the eyelids to rotate the lens into the proper orientation. Gravity plays no role in the latter.

CalMeacham
01-18-2006, 06:16 PM
QED's statements about toric lenses are correct -- a toric prescription has sdphere, cylinder, and the direction of the cylindrical axis as parameters. They're marked on the box. The test equipment we made tested for all of them, angle included.

Toric contact lenses are slightly "bulgier" on the bottom to correctly orient them -- but it isn't gravity that does the orienting. It's drag from your opening and closing eyelids. So you can wear toric contact lenses, and they'll always be properly oriented -- even if you're standing on your head.

Gozu Tashoya
01-18-2006, 06:30 PM
As a physics teacher, I should probably know this. However, I am still confused by how, exactly, a contact lens gives you better vision. My understanding is that it is not an actual lens in the sense that a glass (or other medium) lens refracts light because of the curved shape and refractive index of the material. Does the contact bend your cornea a bit, squeezing your eye to the correct shape? I'm curious. Thanks!

-Tofer
Just wanted to add that there are, indeed, contact lenses (and it'd be interesting to see if these are lenses in the technical sense) that are designed compress your cornea back into shape. (Sadly, they aren't available to people who are as nearsighted as myself. :( )

About ortho-K contacts. (http://vision.about.com/od/contactlenses/a/orthok.htm)

Pleonast
01-18-2006, 06:39 PM
Soft contact lenses follow the contours of the eye very exactly and as such cannot be prescribed for people with astigmatism.Not true. I have astigmatism. My prescribed contacts are soft. They do not correct my astigmatism, but it's mild enough that I can manage.

tofergregg
01-18-2006, 09:00 PM
Thanks for all the info, and I certainly learned something from this discussion. Cheers!

-Tofer

naita
01-19-2006, 05:31 AM
I only used them for a while, but I had soft contacts correcting for astigmatism. And they _did_ have to be oriented the right way to work. A couple of blinks and they were in place. I didn't experiment with holding my head sideways to check how fast they'd rotate out of place though...

And just as an example of my degree of astigmatism: When I got new glasses last year I thought there was something wrong with the new ones. I just couldn't focus on anything. It was just slight fuzziness, not like without my glasses, but it was significant and I said so to the clerk. She of course was used to people finding their new glasses slightly different and wondering if they were right, and told me to try them out for a day, and come back if things didn't get better. After wearing them for an hour I just had to switch back to my old glasses, and even though I tried the new ones on for long periods that evening, they just didn't work. So I brought them back, and the optician briefly checked my eyes again, and measured the lenses. Lo and behold, the grinder had messed up the order and the cylinder was 90 degrees off.

Short explanation: yes, I'd notice if the contacts didn't correct for my astigmatism.

CalMeacham
01-19-2006, 07:02 AM
I only used them for a while, but I had soft contacts correcting for astigmatism. And they _did_ have to be oriented the right way to work. A couple of blinks and they were in place. I didn't experiment with holding my head sideways to check how fast they'd rotate out of place though...



As I note above, they won't -- it isn't gravity that keeps them oriented, and gravity won't rotate them out of orientation. The lens doesn't rotate freely enough on your eyeball for gravity to be a factor. It's the drag from your eyelid "wiping" across your eye up and down that orients the contact lens, and every blink resets the position. You could try not blinking for a long time while upside down, if you want, but I'll bet you don't see any efect.

naita
01-19-2006, 09:02 AM
As I note above, they won't -- it isn't gravity that keeps them oriented, and gravity won't rotate them out of orientation. The lens doesn't rotate freely enough on your eyeball for gravity to be a factor. It's the drag from your eyelid "wiping" across your eye up and down that orients the contact lens, and every blink resets the position. You could try not blinking for a long time while upside down, if you want, but I'll bet you don't see any efect.
Sorry for not noticing that. :smack: Someday I'll learn not to skim... or maybe not.

butler1850
01-19-2006, 11:19 AM
About ortho-K contacts. (http://vision.about.com/od/contactlenses/a/orthok.htm)

Sorry to hear you can't use them.

My buddy was in on the trials of them, and thinks the world of them. Better than sliced bread, toast, and fresh creamy butter served in bed.