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If you see through small enough aperture, you can see things without glasses.

How does this trick work?

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It's basically equivalent to squinting, too. – David Z Nov 18 '10 at 2:30
for me and many others with non-linear glasses it doesn't work at all :() – jwenting Apr 19 '11 at 9:36
I think this works with short-sightedness only, right? – M.Sameer Jun 17 '11 at 11:38
It should work for far-sightedness as well, but not for astigmatism. – Peter Shor Jun 17 '11 at 12:49
up vote 18 down vote accepted

For someone with perfect vision, the lens in their eye focuses a point source in their field of vision to a point source on their retina. For someone with less than perfect vision, the focus lies either before or behind the retina, resulting in a large spot on the retina, instead of a point. See the top illustration.

Nearsighted eye with and without aperture

When you squint, or roll your fingers into a tube in front of your eye, you're placing an aperture in the way of the light. See the bottom illustration. As the diagram shows, the focus stays the same distance away from the retina, but since the angles are smaller, the spot on the retina is also smaller. A smaller spot is closer to a point, and therefore seems "sharper" - even though the vision is just as bad.

The camera obscura works on a similar principle.

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Now this is explanation from "the book"! – Pratik Deoghare Nov 19 '10 at 5:59
When photographers describe this principle they use the terminology: "The larger the f-number, the larger the depth of field." This is a nice explanation of why this principle is true. – Steve B Jun 5 '11 at 6:57

Basically, what your eye does is take an average value of all the light that comes into your eye, and present it as the final image.

Let us consider a wide open eye. This eye receives information from all angles possible, and the larger angle the incoming angle has with respect to the normal line of the eye, the more refraction it will go through.

When your eye does not function well, like myopia, your lens will not be able to refract properly. And every light that needs a refraction will undergo some deviation from its correct path.

What you do when squinting, or the tactic shown in Wikihow, is basically eliminating the larger angle light rays, while only receiving those that require little to none refraction to reach the optic disc in your retina.

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Your answer is correct, but I think there's a more intuitive way to explain it. – ptomato Nov 18 '10 at 23:03

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