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How does squinting alllow one to see clearer pictures? What are the harmful effects?

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This is an optical pinhole effect. If the amount of light passed through the lens decreases, the sharper the image is at the focal point. For a camera this is accomplished by increasing the f stop (f22 is a smaller aperture size than f6 — f-stop is an inverse number). For the eye the iris will dilate (get larger) or contract to be smaller. In very bright light, you can cut down on large amounts of light coming in by squinting, which creates a slit in front of the eye lens.

In photography this increases the "depth of field". Which is to say that more objects are in focus and sharp. It works the same for our eyes of course.

aperture effect

In this image you can see as the aperture is smaller, any given point on the focal plane (right hand line) receives light from a smaller angle (i.e. from less of the image field).

I am not a physicist, but that's a first order explanation @Luke reminds me that there was a question about whether squinting is harmful. There is another reason that we squint. If we are nearsighted or if we are getting older, we have a tendency to squint not because there is too much light coming into our eyes, but because our lenses are not focusing very well. Is it harmful to strain our eyes? Maybe, but it won't kill you. Reading glasses won't either.

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I can't think of any harmful effects, can you? Maybe constantly squinting would actually alter the shape of your eye/retina slightly, thus meaning you'd always need to squint (or get glasses)? –  Luke Oct 5 '12 at 15:48
    
@Luke i added some thoughts to the answer. I think eye strain that results from squinting when your eyes are not really seeing things well might tend to hurt your eyes. squinting because the light is too bright is probably a good thing and might be as good or better than sunglasses. –  shigeta Oct 8 '12 at 17:01
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I think there is more to it than the pinhole effect. Have you ever noticed that you can make out faces by squinting your eyes at a pixelated video of someone.

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please add some details and some justification based on reasoning and/or references. –  WYSIWYG Feb 21 at 8:33
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