I was just wondering, why does squinting help me see without my glasses?
Yeah, there’s not much more too it than that.
Oh god… that should be “to it than that.” I always criticize people for that.
I’ve always assumed that the pressure changes the shape of the lens a little bit, but I’ll admit that’s just a guess.
You can also improve nearsightedness by peeking through a very small hole (such as is formed by curling your forefinger and thumb), but I’m pretty sure that works by a different mechanism, since it requires a very small opening, and squinting doesn’t.
Something like stopping down the aperture of your camera diaphragm; it gives you greater depth-of-field, I think. Just a WAG.
Same deal of you make a pinhole in a piece of cardboard and look through that.
You can make two pieces of cardboard with pinholes and use them instead of expensive glasses. 
When you squint, you’re doing more than just closing your eyelids. You’re also tensing the facial muscles around the eyes. This tension squeezes the eyeball a bit, changing its shape and possibly your cornea’s and lenses’ shapes.
All these shape changes allow the focal point inside your eye to change so that it’s closer to the retina. The closer the focal point to the retina, the clearer the image.
It’s not a perfect fix, as you tire from squinting pretty quick. Also, if you’re like me, no amount of squinting will give you good enough vision to drive. Me, I’ll stick with glasses.
(bolding mine, of course)
:eek:
I never knew that! But darned if you’re not right. In fact, it works better than squinting. If I take my glasses off, I can see only white spots on my calculator where the keys are. While squinting, I can see black squiggles on them, but not make out what the are. While looking through a teeny hole made by curling my forefinger to my thumb base, I can see numbers on the keys well enough to read them!
How the heck does that work?
Waitaminnit. I just discovered something weirder. If I take off my glasses and put a finger near my eye (closing the other eye) I can see a line of in-focus just close to my finger and read the calculator keys just by holding my finger close to my eye - not looking through a hole! What’s going on here? Is my “aura” giving me better vision? :dubious:
I think this explains it fairly well. It’s not got to do with changing the shape of the eye of any of its compnents.
From KarlGauss’ link: “But anyone who’s farsighted or nearsighted will see a little better looking through a pinhole. (We routinely do it on examinations). The effect is even more dramatic if you have astigmatism.”
That explains me. Very nearsighted, with astigmatisms that go the “weird” way. (Horizontal where most are vertical, or vice-versa. I never remember which.)
This question came up a while back, and CalMeacham, who is a stone expert at optics (and also, apparently, 50’s vintage SciFi), is going to come into this thread at any moment to tell you that the aperture/pinhole idea is correct and the reason is that it reduces off-axis aberrations. A light ray coming into a lens dead center perpendicular to the surface will focus perfectly (of course, that’s only a single point on the focal plane). But the farther off the axis the ray is, the more out of focus it is. CalMeacham will have to explain why if you want to know.
I had assumed that the pinhole effect was due to increased depth-of-field as stated earlier. But apparently that is (as already pointed out) a different mechanism. On one of my first eye exams, the optometrist shone a bright light in one eye, and suddenly my vision in the other eye became much sharper. The optometrist agreed with me when I guessed that contracting pupil = smaller aperture = increased d-o-f.
As for WhyNot’s finger trick, I’d guess that the edge of the finger acts analogous to an aperture.
Awwwwww…
CookingWithGas, you’re so nice. You’re right about what I’d say, and with whast others have said. But I’m really here because this ties in so well with the column I wrote for this month;s issue of Optics and Photonics News. Have a look:
http://www.osa-opn.org/view_file.cfm?doc=%(%2C%2F%2BJ<%2C%20 &id=%(\%2B*K%2C0%20
Klondike Geoff, someone proposed exactly what you do in the American Journal of Physics (the Mathur and Bahuguna article listed in my bibliography). Thety were speedily answered by another article in the same journal by an eye professional who pointed out that , while the pinholes would allow you to see clearly with either eye, using both eyes causes your eyes to accommodate to make the images coalesce, and that imposes its own strain, especially if the pinholes don’t line up just right, so such pinhole glasses produce their own kind of strain.
The issue of pinhole glasses is interesting, and I’ve got so much on them that want to do a follow-up on the ones they sell for about $20 on the internet and late-night TV. The quick answer is that they kinda work, bnut they’re not magical devices, don’t correct as good as properly done glasses, won’t fix your eyes, and ave their own brand of eyestrain. It’s not because of some evil conspiracy of lens manufacturers and eye doctors that these are not prescribed.
Don’t contacts change the shape of your eyes? I thought the reason I had to stick with rigid gas-pearmeable for so long was because the soft didn’t change my eye shape correctly. Even now I only have 99% vision with the soft contacts…but it’s better than those horrid rigid ones.
I Am Not An Ophthalmologist, but I think the answer is “no”. There are, I understand, some items that are worn to affect the shape of the cornea, but your garden-variety contact, whether hard or soft, doesn’t do this. They base the design and prescription of the lens, after all, on the error in vision that they measure when you haven’t got any glasses on. It would be much more complicated if they then distorted your cornea, giving a different power completely, then had to correct that new power with the contacts.
I’ve yanked out my copy of W.A. Douthwaite’s “Contect Lens Optics and Lens Design”, which shows a design excample where the designer deliberately uses a back radius of curvature of 8 mm on an eye that has a 7.8 mm radius of curvature. He’s clearly trying to match the curvatures, not reshape the eye.
On p. 235 it says that “If a soft contact lens is placed on a particular eye it will warp so that the back surface takes up the curvature of the cornea on which it rests.” Soft ones, in other words, won’t change the shape of your eyes at all.
I’ve designed and built devices for measuring bioth hard and soft contact lenses – the power they give on the package is really the optical power of the hydrated lens, so there’s no “fudge factor” in there for the warping of your eye lens. (There is a slight allowance for the effect of the layer of tears between the contact lens an the cornea, which may be why contact lense prescriptions are often slightly different from glasses prescriptions, but that’s different.)
One important issue that has not been pointed out: if you’re going to try the pinhole experiment, don’t hold the cardboard in front of your eye while you’re punching a hole in it.
Unless you’re drilling the hole in the cardboard from the eye side, using your heat vision.
You laugh. However, I once explained this to a woman I used to work with. She held up a sheet of paper in front of her face and asked me for a safety pin.
I laugh more after that.
I’m going to have to resurrect this ancient thread.
I just noticed that when I put my glasses on when squinting, it would blur my vision. If the pinhole theory is correct, putting on my glasses shouldn’t do this.
But this would support the eyeball/cornea/lens deformation theory. Since the deformation of the lens has corrected my shortsightedness, additional corrective lens would over correct and thus result in farsightedness.
The pinhole theory is correct; the deal is, if you’re squinting, to improve your vision, then you put on your glasses, the pinhole is no longer correctly arranged. It turns from being a “good” pinhole to being a “bad” pinhole.
Suppose, for sake of argument, you had a pair of glasses that belong to someone else. They aren’t your prescription. You could squint through them, to improve your vision. Without them, you can also squint to improve your vision. But the two squints are the same: one would be tighter, or deeper, or narrower, or…“squintier”…than the other.
Squints come in prescription grades too!
Another pinhole effect is to hold two thumbs and two forefingers together, to make a very small hole – a pinhole. Look through it, and adjust it, tighter and looser, and you can improve your vision. (A little.) The same is true for it: the size of the pinhole changes depending on how much vision-correction you need.