Your lenses suck at focusing, that's why you need glasses. Focusing just means bending the light to go to a smaller point at a particular distance. Do you know what light doesn't need to be focused? Light that's already on its way to that point. So what happens when you squint? You're actually removing all the light that requires bending so what's left is the light that would already hit your retina correctly. You're effectively turning your eye into a pinhole camera

It slightly flexes your eye ball and change it's shape, which is essentially what corrective lenses do

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You can attribute it to a few things. Astigmatism correction due to your eyes being compressed into a rounder shape at your lens, like squeezing a slightly oval balloon from the tied and opposite ends, the deepening of your lens due to the slight compression creates a stronger magnification, and your lashes reduce ambient light that could be creating a haze over the object you're focusing on.

I didn't realise the muscles around the eyes would be strong enough to squidge the eyes slightly into a different shape. Kind of icky but interesting to know! That all makes a lot of sense, thanks!

The smaller the opening, the less the shape of the lens plays a role. This is the same as using a pinhole camera instead of a lens to focus a photograph. This is also why bad eyesight is less of an issue when there's plenty of light, as then as a matter of speaking your iris is doing the squinting.

So why does it become less focused when you're, let's say in a room and squinting at a PC screen?

Thank you, this makes a lot of sense! This is ringing some bells to me, suspect I did learn this back in class many (many) moons ago.

you are using muscles to squeeze the eyeballs into a different shape that focuses better. Constantly tensing the muscles can make them sore

If you have a poorly formed lens in your eye, it won’t bend light correctly. By squinting, you focus the light like a pinhole camera rather than relying on your lens.

https://i.imgur.com/PiMO3n6.jpg

Your eyes get tired because you’ve got muscles that flex your lens, and they’re working overtime to try and fix the focus on your poorly shaped lenses.

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