Astigmatism: how does correction work?

Astigmatism is visual distortion caused by a misshapen cornea. Rather than round, the cornea is ovoid or almond-shaped. Does anyone know exactly how this effects light travelling through the cornea how the optical defect is corrected with glasses and contact lenses?

PS Here’s a lame web page about astigmatism

Astigmatism is cause by the warping of the cornea, one reason why it occurs as you age. The cornea’s surface becomes “wavy” and the image is distorted.

With glasses, they are designed so they have a distortion “opposite” of those on your cornea.

It’s even easier with contacts – your tears fill the space between the cornea and the contact lens, making them, in effect, one unit with the contact lens as the surface. The first contacts were to cure astigmatism.

Here’s the defintion at Britannica.com

Apparently there are no eye doctors on this board. Astigmatism, as you know, does not occur in older people, but is a life-long problem. If you have no astigmatism, your correction, as your link pointed out, consists of only one number, the diopters, either negative or positive. If you have astimatism, there are three numbers. The 2d, is the refractive correction for the astigmatism, expressed either negatively or positively. The 3d, and I’m not all confident about this, is the “globe” or a circular measurement needed. I’d really like to know about that 3d number, too.

I used to play tennis with an opthalmologist, but I never asked him that question. However, he did tell me that astigmatism will account for 50% of nearsightedness. In my case, I had RK, and the myopia in my left eye is plano. However, due to the surgery,the astigmatism is -3. This doctor said that that makes my myopia -1.5, although without the astigmatism I would have no myopia.

This probably does not answer your question, but since no one else has, I thought I’d tell you the little I know about it.

Actually, truly correcting astigmatism with contact lenses turned ou toe a real bear of a problem. You have to make a lens for each eye that cancels out the “warping” effect. This means hat you have a lens which has one “power” along one axis, and a different “power” along the axis perpendicular to this. The actual powers required, and the directions they lie along are different for each person’s eye. Making eye glasses that correct in ths way is relatively easy – you can grind the lens in th same way you make cylindrical optics, then permanenly mount it in an eyeglass frame so that it is oriented the precise way t needs to be in order to compenate for the eye’s defects.

The first problem with making the contact lens is making the correct on such a small and thin piece of plastic. Current hydrophilic lenses are cut individually on a lathe with piezotransducers mounted on it, or ae molded from astigmatically-ground molds, or ae molded in a “compressed” state, then released.

The second problem is making the contact lens orient itself correctly on the eye. If you just use an ordinary circular lens it can orient itself in any direction. But it won’t do any good unless it is oriented to compensate for the defects in the reye. It was eventually found that if you made th lens so that it is thicker on the bottom it wil tend to remain oriented that way. It’s not that the thicker botom is pulled down more by gravity – it’s that the motion of the eyelid as it closes tends to push that thicker portion into the downward direction.

I know whereof I speak – I used to work on equipment for these contact lenses.

Contacts I can understand. Your eye needs the light bent different amounts depending on which part of the cornea it comes through, so a contact that covers a cornea, can correct for it.

But if the amount of correction changes as move laterally (peripheral vision requiring different correction than straight ahead vision), how does a glasses lens do that? When you look straight ahead, it’s adapted to the differences, but when you look towards one side (even a little bit) aren’t you now looking directly through a part of the lens that is set to correct for more peripheral vision?

From my wife, an optometry student (everyone say hi to Lisa!)

Astigmastism can come from a few different places, but the most common one is the cornea. The cornea has 2/3 of the refracting power of the entire eye, which is around 45 diopters. The cornea is generally spherical, but with astigmatism, there are actually two different powers in two different meridians, almost always 90 degrees apart and most commonly at 90 and 180 degrees. The vertical meridian usually has more power (with-the-rule astigmatism). A theory for this is that all of the blinking of the eye can very slightly make the vertical meridian more concave, giving it more power. So with astigmatism, there are two focal points aimed for the retina. They can be both in front of the retina (myopica astigmatism), both behind (hyperopic astigmatism), or straddled around the retina (complex astigmatism).

There are many options for correcting this optically. One can use a “spherical equivalent” which does not correct the astigmatism, but simply centers the two focusing points to straddle the retina. Many people with slight astigmatism do this. Both meridians can easily be corrected with glasses. Spherical rigid gas permeable contact lenses (hard) can correct up to 2 diopters of astigmatism, because the tear film will fill in the difference and act as refracting material. Conventional soft contact lenses will not correct astigmatism, but now toric soft contact lenses are common. These will have a thicker prism ballast in order to sit correctly on the cornea.

So typically this astigmatism that I discussed develops at a young age (as with any refractive error), and can progress through the teens. Astigmatism can also come about from a lens that is not spherical, or other corneal abnormalities, such as corneal dystrophies (occur with age), laser surgeries gone wrong (such as barbitu8’s RK), etc.

The three numbers that are used in the prescription are the spherical power, the cylinder power, and the axis, which lets you know in what orientation the spherical power should sit.

Peripheral vision requires the SAME correction as central vision. You always look out of the center of your cornea - the visual axis is always through the center. As your gaze changes, so does the position of your cornea. You are correct that only the center of you glasses’ lens has exactly the right correction. When you first get your glasses, everything in the periphery may be distorted, but assuming you have about equal refractive error in the two eyes, everything is distorted the same for the two eyes, and people adapt in hours to days. We have perceptual constraints that tell us how we should perceive things, so if something in the periphery appears flattened, for example, we know that the actual object is not flattened.

This issue is pretty much alleviated with contact lenses, because the center of the lens moves with your visual axis.

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Hi Lisa!

Man, if ever a diagram would help, this is the time. Are there any linkable ones? Something with light rays, corneas, and retinas?

Hi, Lisa, and thanks Edwino for the info. I’m with ChoosyBeggar. A diagram would be nice. You got to draw me a picture.

By the way, RK is radial keratotomy, which is not the laser. It predates laser. Laser is PRK (photorefractive keratectomy). And I don’t think the surgery went wrong. Everybody said the guy did a great job. Even the ophthamologist I used to play tennis with and who didn’t like the guy who did the surgery said so. (He also said that he would not have done the surgery with my vision.) I needed 16 cuts in each eye. They only can do 8 now (if they ever do that now), and I think that’s by law. It’s just that my vision was so bad, astigmatism developed - I guess due to the number of cuts. But, hey, I’m plano now in the left and -2 in the right, with -3 astimatism in the left and -1 in the right.