Squinting your eyes often makes objects appear sharper, even more so if you’re shortsighted.
It’s because the rays are focused on the retina more than a change in the shape of the eyeball.
Ideally, the eye receives rays of light and bends them so that an image is resolved on a small point of the retina.
If the rays focus in front of the retina, the person is nearsighted and cannot see distant objects clearly. If they focus at a point behind the retina, the person is farsighted, and nearby objects are blurred.
The shape of the eyeball and the focusing power of the lens and cornea help determine focus, but the angle at which light rays hit the eye plays a role.
Light comes into the eye from all directions.
Rays entering the eye at an angle from above or below would tend to focus somewhere before or behind the center of vision.
Those rays coming in essentially perpendicular to the eye, on the other hand, would tend to be focused more directly on the retina, providing a clearer image of what one is looking at.
The basic impact of squinting is to reduce the number of superficial or peripheral rays of light that enter the eye, so only the rays coming directly in are focused on the retina.
This cuts out a lot of the rays that are out of focus and eliminates a lot of what would otherwise be a blurred image.
You can’t solve vision problems by squinting your way through life, but squinting might help someone who has lost his glasses and needs to see a road sign.