My glasses broke earlier today, so I’ve been squinting quite a bit. Which got me to wondering: why does squinting make you see more clearly?
Because it makes your eyelids act as a small aperture. This means that only light rays that are nearly parallel can get though the squint, and these light rays all focus at the same place, regardless of the object’s distance, like a pin-hole camera.
Because you’re restricting the image to the light that comes straight into the eye, with less that has to be focused by the cornea.
Least that’s what I seem to recall my high school physics teacher telling me.
ETA To add, beaten to it but at least I was right, ha.
Yup, pin hole effect.
If you have 20:200 vision, you can still see the time on the LED clock across the room at night, without glasses, by curling your index finger until it makes just a teeny hole surrounded by finger, and looking at the clock through that hole.
Ha, I do this all the time!
On the one hand I understand this, but on the other it seems strange that less light and less of an image results in a clearer image instead of a fuzzier image.
The fuzziness is because the cornea/lens are supposed to bend light so that a large amount of light is focused on a small area of the retina. When your vision starts to go bad, the focusing is not done as well - light from one edge of your eye is focused on a different place than the light from the other edge of your eye. Some people with bad vision (me, for example) actually see multiple double images or a halo around things because of this. By narrowing the light to a pinhole, the lens and cornea don’t have to focus anything - the light passes straight through (or it is curved, but it’s all curved the same). Either way, the light’s all hitting the same spot like it would if it was being focused correctly.
You’re right that less light gets through, but the brain is funny about being able to correct for that. Your brain is more worried about contrast between things than the total light level.
I’m able to get the same effect by holding my index finger up against my face in front of my eye and looking across the room past the very edge of the finger. I found this by looking at the clock on a very far away wall in jail while holding my face very close to the bars of my cell. 
Although it might work in theory, I am not convinced that what people call squinting actually produces a pin-hole camera type of effect, which would require most of the pupil, except the very center, to be covered up. I suspect that what is really happening is that you are putting pressure on the eyeball, thus distorting its shape slightly and changing its focus.
test with a pin hole or pencil/pen tip hole in a piece of paper. it works.
there have been glasses sold with opaque lenses with small holes punched it. which one you looked through you saw a clear image you wouldn’t see with your naked eye.
No – it’s hard to distort the eyeball. You really are aperturing the eye down.
And, yes, it does let in less light, which can be a problem. Lenses are a way to both correct the vision and let in more light.
I did a column on this a couple of years ago. The idea of using opaque “glasses” with pinholes in it goes back to India, but no one seems sure how old the idea is. I’ve got a picture of some old “pinhole” glasses frokm India, but they’re not dated. I’ve made such glasse myself out of cardboard, and they do work. They also sell them on the internet, but they’re not a great substitute for glasses (and shouldn’t be used by children – researchers have found that it can cause problems for developing eyes).
I suspect these might have been used widely in the ancient world – they’re a lot easier to make than glass lenses, and they don’t break. But if they were made of bone or wood or leather, they would have deteriorated. And no one, as far as I know, has written about them.
Squinting is a form of focusing.
The same effect applies in photography. Smaller apertures (larger f-numbers) give you more depth of focus.
The smaller aperture idea also explains why brighter light helps to bring things into focus - you pupil constricts and works just like the pinhole concept mentioned.
Here is a graphic that shows how the aperture effects the way light passes through a lens and its effect on focus. This is in reference to depth of field, but the principle is the same.
With larger apertures, the light is coming in from wider angles, so any error in focus results in larger circles of confusion.
Of course, an aperature need be neither very small nor very round to produce a decently focused image:
Eclipse shadows
I like this one of palm leaf shadows during an annular eclipse:
http://www.eclipse-chasers.com/e10/pics/aseGill03.jpg
from here
This is why I never bought that Twilight Zone episode where the guy is left in the library and he breaks his glasses. Make a pinhole, dude!
It isn’t the pin hole effect. As a poster above said, when you squint the musculature surrounding your eye distorts the shape of your eyeball, focusing the light more effectively on the retina.
People with myopia, or near sightedness, often have long eyeballs. Squinting compresses the eyeball, bringing it closer to its optimum length. This is why squinting does not work for far sighted individuals, or for those with corneal imperfections.
Don’t believe me? Press on your eyeball with your finger. That works just as well as squinting, although you need to fiddle with it to get the focus just right.
If you still don’t believe me, have a friend measure how large the gap is between your eyelids when you squint, and cut out a piece of cardboard or whatever to that width and look through it. If the pinhole effect is at work, you should notice an improvement in vision using that cardboard slit.
I’ll be damned. I’m blind as a bat without my glasses and I’ve never tried this. Here I am holding my fist up to my eye and reading the calendar, the monitor, etc . . .
My girlfriend learned this trick a few years ago. I wish she hadn’t. It can be quite embarrassing in a restaurant or anywhere else when she makes a very tiny “O.K.” sign and holds it up to her eye. She does it constantly when texting (which she also does constantly), checking after every few letters to see what she has written. While her glasses are parked on top of her head. In spite of that, she is famous for sending texts that are virtually unintelligible. A cashier saw her reading the ingredients on something while using this method and assumed she had a disability of some sort and rang her up as if she were a five year old. To the guy that taught her this - I will hunt you down and mess up your hair.