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Here is a small experiment my tutor once told us for just amusement. It works for myopic people at least, and can be a good check to see if you have myopia.

With your naked eye, ("remove the spectacles" for myopic spectacle wearers) see a considerably distant object (a few meters). It should be a bit blurred and unclear if you have myopia. Then, using your two thumbs and first fingers of both hands, create a tiny square in between. Using a single eye, look through the square. For a large square, the image is still blurred. But as you purse your fingers tightly and reduce the dimensions of the space between, you will eventually reach a point when the image comes in perfect focus. The image is perfectly clear as you would see it with spectacles. For an object 1-2m away, the square needs to be much smaller than that in the picture.

Not my handsNot my hands

This works for almost any myopic person. Why does it work? Does diffraction come into play? If it does, I don't see how that can adequately refract rays to act as a lens?

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No, it's not diffraction. It's because you increase the depth of field by reducing the aperture.

Basically, the more concentraded the light is, the better you see a defocused images.

See this question: Why does aperture size affect depth of field in photography?

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  • $\begingroup$ aka. squinting. $\endgroup$ – user6972 Nov 21 '13 at 7:36
  • $\begingroup$ By putting a small aperture in front of your eye you are in effect making a pinhole camera. One of the answers to the question jinawee linked includes ray diagrams showing how a pinhole camera gives a sharp image. $\endgroup$ – John Rennie Nov 21 '13 at 9:47
  • $\begingroup$ From what I read, the sharpness is caused by elimination of various spherical aberrations. But in case of a myopic person, the normal blurring is due to the extra power of the eye-lens to converge rays. How is that corrected by this pin-hole effect $\endgroup$ – Satwik Pasani Nov 22 '13 at 9:00
  • $\begingroup$ @SatwikPasani sharpness is caused by elimination of various spherical aberrations. This effect is not important in the eye case and not much in the camera case. $\endgroup$ – jinawee Nov 22 '13 at 9:27

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