# Why nearsighted people see better with their glasses *rotated*?

If you are nearsighted (like me), you may have noticed that if you incline your glasses, you can see distant objects more clear than with normally-positioned glasses. If you already see completely clear, you can distance your glasses a little more from your eyes and then do the stuff. To do so, rotate the temples while keeping the nosepads fixed on your nose, as is shown in the figures.

As I said, starting with your glasses farther than normal from your eyes, you can observe the effect for near objects too. (By distant, I mean more than 10 meters and by near I mean where you can't see clear without glasses)

Note that if you rotate more than enough, it will distort the light completely. Start from a small $\theta$ and increase it until you see blurry, distant objects more clear. (You should be able to observe this at $\theta\approx20^\circ$ or maybe a little more)

When looking at distant objects, light rays that encounter lenses are parallel, and it seems the effect happens because of oblique incidence of light with lenses:

$\rightarrow$

The optical effect of oblique incidence for convex lenses is called coma, and is shown here (from Wikipedia):

I am looking for an explanation of how this effect for concave lenses (that are used for nearsightedness) causes to see better.

One last point: It seems they use plano-concave or convexo-concave lenses (yellowed lenses below) for glasses instead of biconcave ones.

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I'd suggest that you update your question to indicate that it may be specific to particular eyes and that some people (e.g. me!) benefit from rotating about a different axis (e.g. the vertical axis). Plus it only works in one direction. Also, note that increasing the distance between the lens and the eye (as per your first paragraph) changes the focal length of the lens-eye system, which may cause a different effect entirely. –  Tom Shaw Aug 2 at 13:10
Part of the missing puzzle is that the real model is a concave lens (glasses) which are tilted into a biconvex lens (eye). I think you'll also find the focal distance changes slightly in this case. In addition there can be an astigmatism (regular or irregular) in your eye that might be improved using this tilted distortion. –  user6972 Oct 2 at 7:37

Your basic premise that one can see better with the glasses tilted is false. If the lenses have the right correction for your eyes, then tilting them will make things worse.

The reason this does work often is that the lenses are not at the right correction. In young nearsighted (myopic) people, the myopia usually gets worse with age. The glasses may have had the right correction 2 years ago, but meanwhile the eyes have gotten a little more myopic and a stronger correction is required.

Tilting the lenses makes light pass thru them in a way that effectively makes the lens seem stronger at that angle. If distant objects are a little blurry due to more myopia than the lenses are correcting, then tilting the lenses will make horizontal edges sharper. It does nothing for vertical edges. But, overall the image will appear sharper.

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This is exactly what I'm asking: How "light pass thru them in a way that effectively makes the lens seem stronger at that angle"? –  Mostafa Jul 14 at 12:35

My theory is that this effect is not seen by most myopes. Rather, it indicates that you and I have an uncorrected higher-order aberration in the eye, i.e. coma, which is corrected by inducing the opposite amount of coma in the spectacle lens by rotation.

I have noticed that rotating my left spectacle lens about the vertical (not horizontal) axis by $10 ^\circ$ in a particular direction gives me much crisper vision. I've also noticed that when I look at a distant circular light source (like a traffic light) with the lens unrotated, it looks somewhat like this image of uncorrected coma. The image is much better with the lens rotated.

Note that the sphere and cylinder prescriptions in the (unrotated) lens were confirmed yesterday by an optometrist as being the best I could get -- but simply by yawing the lens I can see much better.

I'd appreciate it if someone familiar with optics could confirm whether or not coma can actually be corrected in this way. Everything I can find online about higher-order aberrations in the eye suggests that they can only be corrected by refractive surgery or contact lenses. In addition, could a lens be constructed to perform the same correction without rotation?

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