system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I am very nearsighted. Why does my vision appear to significantly sharpen when I squint?
gpt
The black circle in the center of your eye (the pupil) lets in a lot of light.  It gets bigger and smaller to let in more or less light.  When your pupil is big, light from a single point in the real world is entering the eye at slightly different angles.  The eye focuses the light from these slightly different angles back into a single point to make a sharp, clear image.  But if you're nearsighted and aren't wearing glasses, light from far away won't be in focus.  Instead of the light focusing to a point, it hits your retina (the part of the eye that senses the light) in a circle shape.  Because of this, you see a blurry image.

When you squint, you're blocking part of your pupil with your eyelids and eyelashes.  Everything looks darker, but the light isn't spread out as much on your retina.

If you take a camera and set the aperture (the camera's pupil) to be big, you'll notice that only a small part of the image is in focus, and everything else is blurry.  Now if you close the aperture to make it smaller, a lot more of the image looks sharp.  This is for the same reason.  Unfortunately you can't control your eye's pupil size  - which is why we have to squint to achieve this effect.

Interestingly, if you look at a bright small light in a photo taken with a wide aperature, you can sometimes see big blurry circles or hexagonal shapes where the light was.  This shows the shape of the aperture and how much the light from a point spreads out on the camera's light sensor.