At least for me, I notice that when I squint, my vision becomes sharper. I don’t know if this is a common thing, but I’m curious as to why it happens
In: Biology
I’m seeing some inaccurate answers here so here’s a more scientific one.
It is the same effect that happens in (actual, not common cellphone) cameras when you use a smaller aperture. It gets you more depth of field, which gets you overall better focus.
In physics there’s no free lunch, so you’re trading depth of field for luminosity (in a camera you compensate by using a longer time of exposure, with the eyes, you pupil will open wider, but if you really squint, the image will be darker).
Now why does that work?
When you focus on something, you’re essentially making light that comes from one exact point in the thing to hit one exact point on the light sensor, or your retina.
You can do that without a lens, by making the light pass by one tiny hole (google camera obscura or pinhole camera). If you trace a line from any part of the scene to the back of the camera (or retina) passing by one single tiny pinhole, there’s only one straight path, leading to one exact point on the other side, right? That means it’s focused, and a pinhole camera will have everything on focus at the same time, near or far.
But having light passing by one minuscule point makes the image on the other side too dim. The solution would be using a larger hole then. BUT you can visualize that now you can have a lot of lines coming from one exact point in the scene passing by that big hole and hitting the back of the camera, so for each point in the scene you now have a circle, not a point, at the sensor/retina. It’s out of focus.
How do we fix that?
With a lens. You get a big diameter lens, and now you get all the light coming from that point in the scene and passing by all the face of the lens to converge on a single point on the sensor. We now have a lot of light, and focus.
BUT. That geometry only works for an exact distance between the object and the lens. So we need to adjust focus (changing the geometry of the lens; using multiple moving lenses on a camera, or reshaping the lens in your eyes) to focus things at different distances.
If your eyes cannot focus correctly (myopia, hypermetropia, astigmatism, presbyopia) then the lens is not making a good job at converging each point in the scene into a single point at your retina.
If you have a LOT of light, your pupils will contract enough that focus will get better (smaller hole, more precise the point at the retina), but it may not be enough. In lower light conditions the pupil will open more and focus will be worse.
So when you squint, you are reducing the “hole” through which light can pass. You are using a smaller area of your lens to collect light and converge on the retina, so the image will get darker, but now each point in the scene will converge to a smaller spot at the retina, and that translates in better focus.
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