I have poor (uncorrected) vision @ about 20/400. I dont know where I learned this, but if I make a pinhole (with my hand) to look through (by curling up my index finger into my thumb) and look through it without my contacts or glasses on, I can see (almost) clearly through it! Has anyone done this, or can anyone explain how that works to me?? I’m figuring it has something to do with light…
The pinhole forms a crude lens. You’re essentially viewing a projection.
Pinhole cameras existed long before glass lenses were used. Poke around under “pinhole photography”, and you’ll find a lot of stuff like this:
Hey, I have 20/400 vision, too. Didja know that we’re legally blind? Yep, I asked the optometrist one time what I was, you know, “20/what?” He had to figure it out on a piece of paper, and after a while he said, “Well, as near as I can tell, you’re about 20/400.” He added, “That’s legally blind, you know.”
I had always known I was tremendously nearsighted. I said, “Oh.” He then felt it was incumbent upon him to warn me, “You know, that means you shouldn’t drive a car without your glasses.” I hastened to reassure him, “Doc, without my glasses, I’d be lucky to be able to find the car in the driveway, let alone back it out into the street and drive off.”
Did you know that they don’t even bother to measure it that far anymore? When I was getting examined for my LASIK surgery last year the doctor was telling me that they now just go by “20/200+” because anything worse that that isn’t an accurate enough measurement to bother with!
I’ve heard though, that being 20/200 after vision correction is legally blind. I guess technically we are blind without correction, but at least we are able to be corrected. I know we could never be FBI agents, since you need to have better than 20/200 uncorrected.
I always knew I had serious issues with my vision. I know I would have a better chance surviving a drive in my car wasted out of my mind than without my glasses or contacts.
But all you blind people, try the pinhole experiment. It works!
That’s a decent link, but the first line there says: “Pinhole photography is lensless photography.” Perhaps there is some other reference that considers a pinhole to be a crude lens?
But the OP’s pinhole made with her hand is just like the pinhole camera, of course. Reducing the size of the opening makes the circle of confusion smaller, just like reducing the aperature of a camera–more things are in focus. Make the hole really tiny–like a pinhole–and almost everything will be in focus.
There’s either one of two things going on here:
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By making a pinhole, you’re eliminating refracting optics altogether. If this were the case, you would probably see whatever you’re looking at as if it were upside down.
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You’re forming an aperture stop that reduces the signal from what you’re looking at, but improves your depth of focus. I’ll bet that this is what’s happening.
Lenses work by collecting the light signal spreading from each point on an object and converging it back to corresponding points (for the math buffs here, it performs a partial fourier transform on the image and a second fourier transform on that. On the other hand, I’m not a math buff, so I just trust the folks who say it does.)
In a perfect world, reducing the numerical aperture of a lens will increase the depth of focus for a lens but will reduce its resolution. My guess is that either the focal point changes from the outside to the inside of your eyes’ lenses or you’re increasing the depth of focus to a point where the middle of your eyes see the image as being in focus.
Not only do pinholes in cardboard or in your hand focus light, but your pupil (the hole in your eye) can work also.
Try this MissM. Go outside on a bright sunny day w/o your glasses and try to read a newspaper. You’ll find that it is clearer than it normally would be. That is because the bright light makes your pupils very small (almost pinhole size) and they aid your cornea and lens in focussing light on your retina
Yes, I do it almost every day.
When it’s about 4:30 AM, my body usually wakes up. I know I don’t have to get up yet, but I’m curious what time it is. I glance at my wife’s digital alarm clock and can’t see it for shit. I make a small hole with my fingers and look at the clock through that and can see the time just fine.
They meant “lens” in the sense of a physical object made of a transparent substance. I meant that a pinhole produces a crude means to project an image, like a lens. It’s a matter of semantics. I will also admit that I was speaking off the top of my head, and may not have been as accurate as I should have been.
I don’t know how or why it works but I do know some interesting tidbits about this phenomenon. (I read how it worked once but do not recall)
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Eskimos used shells with small holes in them to correct vision. (This might not be true, but I remember reading it and it sounds good.)
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Almost everyone has used a principle very similar to the pinhole method of vision correction. It is called squinting.
Not to split hairs, but this is an aperture, not a pinhole. I’ll try to find illustrations if anyone wants.
Yeah, its like an aperture, you know when you step down the F Stop on a lens you get more depth of field.
BTW 20/20 isn’t the best there is.
The effect is nothing to do with lenses - semantic or otherwise. This is a form of Fraunhofer diffraction which exploits the Fourier relationship between spatial amplitude and frequency.
Quoting bits from Optics by Hecht and Zajac which was one of my standard Physics texts.
So when you view something through a pinhole, you are constructing a spatial fourier transform - which sharpens the image considerably.
Russell
20/200 may be a way of speaking but it is quite meaningless.
When you go to get your eyes checked for a prescription you get a prescription that states the power of the lens in diopters, both spheric and cylinder, distance between centers etc. This is what counts and what you need to make your eyeglasses.
20/200 is just a rough way of speaking. It tells you roughly how good or bad your eyesight is but nothing more.