Why does squinting improve vision

Yes and it has the same tradeoffs. The fingers pinhole sharpens the image but also blocks out a ton of light making everything darker. A camera’s f/20 aperture (small hole) will let less light than f/1.8 (large hole). The camera’s mechanics can compensate for f/20 by lengthening the exposure time. The human eye can’t do that so everything just stays darker.

Wait, you mean I can actually buy, fairly cheaply, a set of those glasses with the tiny pinholes all over the lens that seem to magically sharpen my vision?!?!?!

Where do I sign up? and how long can one use them without damaging your eyes even more?

Back in the old, old days people used to play around with pinhole cameras. You could build a camera with no lens other than a pinhole. Rig it so that you can open the pinhole, expose the film and you could get some amazing pictures. Of course, it took some practice to get it right but focus wasn’t a problem.

I don’t want to get into the technical issues of photographic lenses but the smaller the aperture (the area that actually allows light through) the greater the depth of field and the better the focus. But, sufficient light is required.

So, when you squint, you are reducing the aperture of your eye. But you are also cutting off your field of view and reducing the amount of light that hits the retina.

Everything is a trade-off and the human eye developed accordingly.

Also, a lot of the recommended devices for viewing a solar eclipse are based on the principles of a pinhole camera and it does work.

Check post #11 - it causes eyestrain to do this. Use the trick for checking your alarm clock across the room in the middle of the night, but get proper corrective lenses for everyday use.

I saw them being sold in pharmacies. They look like (Google image search pinhole glasses). Apparently advantages are: slows myopia progression. Disadvantages: everything looks darker.

I think I can answer this factually: it’s simply to keep your line of sight from wavering too far from the target. Depth and distance resolution are taken care of inside your eyes. Squinting is not analogous to reducing apperture size to lessen light disturbances (you get better results by wearing a baseball cap or shading your eyes with the flat of one hand on your forehead.)