Pinhole camera: How "How to See Without Your Glasses" works?

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.

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.

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.