So I, like most other astronomy enthusiasts, have, at some point, not been able to see something clearly, such as a star, and have had to squint to be able to see it better. When this happens, the object seems to become clearer, but the light seems to gain a cross, or sometimes a hexagon, or an irregular line shape when doing so. I've noticed a similar effect on different models of telescopes, especially the newtonian reflector series. More on this here: (http://www.youtube.com/watch?v=ipe3NN1yPzM). Is this a similar effect on the eye? But more importantly how does squinting improve the quality of the picture?
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Squinting works the same way as a pinhole camera.
Ideally, light from a single point source entering your eye anywhere on your pupil will be focused on a single spot on your retina. But this works perfectly only if you have perfect vision; otherwise light entering near the top of your pupil may be directed to a slightly different spot on your retina than light entering near the bottom.
By squinting, you block out some of the light from the edges, effectively making your pupil narrower, creating a sharper but dimmer image on your retina. (You may find that it improves the vertical resolution more than the horizontal resolution.)
If you happen to be nearsighted (as I am), you can see a similar effect by looking through a small pinhole, or through a small aperture made with your fingers. If the light is bright enough, you'll see a dimmer but sharper image.
The irregular shapes may be interference from your eyelashes.
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$\begingroup$ +1 and from this same effect cometh the pinhole glasses :) $\endgroup$ Oct 8, 2013 at 2:09
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$\begingroup$ Is this right? Isn't it that when squinting you are only using the center of your lens which focuses light better than the edges of your lens. This is why when using a camera there is an optimal aperture to use. If you close the aperture too much then you begin to lose sharpness due to diffraction effects. Referring a pinhole camera does not seem to be quite correct since a lens is involved. What do you think? $\endgroup$– ehsteveOct 11, 2013 at 20:37
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$\begingroup$ @ehsteve: Well, I think that my answer is correct -- but I'm not an optics expert, and my answer is based just on my own common sense and personal experience. I do get a sharper image either by squinting or by looking through a small aperture made with my fingers. And I think the pupil is substantially smaller than the lens (at least in daylight), so you're not really using the edges of the lens anyway. I think that squinting combines the effects of a pinhole and a (likely imperfect) lens. But if someone who actually knows this stuff has better information, I'd be glad to hear about it. $\endgroup$ Oct 11, 2013 at 21:12