Why does squinting often result in better focus? Is it similar to the pinhole camera effect (the smaller the aperature, the clearer the focus)? Also, I’ve noticed that if I bring my finger up right next to my eye, and focus on something several feet away, there’s a fuzzy region around my finger, and anything that moves into that region actually becomes clearer. Is this related to squinting?
With squinting, your actually changing the shape of your eye (not sure which part, if it’s just the cornea or the whole ball) just enough to affect the focus.
The fuzzy region is because you can only focus on one distance at a time. When you bring something new to that area, your brain sees something new and tells the eye to re-focus on that instead. Notice that your finger that was fuzzy is now focused, and the background several feet away is now fuzzy.
No, the finger remains fuzzy, and the background becomes clearer.
I am terribly near-sighted (Hey, I can still read without my glasses, what more do I need?) but I can give myself a rather noticeable vision improvement by pushing on the side of my head. It’s almost like I’m pushing on bone and compressing it. Very odd.
You’ve got it. The pinhole camera makes every point of light land on a single point of the retina, for a perfect (inverted) image. If the hole is big, the light lands as a pattern of that shape, usually just larger, overlapping dots. On a camera with a “fan” iris, the image on the film is that shape, often a pentagon or heptagon. This results in those geometric sun spots on lots of photos taken toward the sun.
Everyone can read better in strong light because their iris contracts and the light is in points instead of big circles.
There’s a crappy pair of “sunglasses” advertised on infomercials that rips people off. They pretend it’s “curing” your vision. It’s made of opaque plastic with lots of tiny holes. You essentially can only see out of one tiny hole at a time, forcing the pinhole focus.
When I have my glasses off or contacts out, I will often view the TV between my fingers. You get the identical effect as squinting, but there is no doubt that the shape of your eye is the same, because you don’t touch it at all.
Omniman, it sounds like you know optics, but I disagree.
My take is that squinting just reduces the stray light entering the eye. Using the trademark symbol in the SD logo at the top of this page as a test mark, my vision gets better if I squint slightly or shade my eyes with my fingers (forming a “tube” with the whole hand works best.) This makes sense, as the glare from the white field of the page is masked out.
On the other hand, if I put the fingers close enough together that they form an aperture, the image gets worse, and ultimately, I start to see interference fringes between the fingers (FWIW, my eyesight is fair, 20/35 or so; I may not be the best test subject.)
This has been discussed before here, more or less. That post was from someone who had 20/400 vision. The pinhole argument was made at the time, but I doubt that we’re seeing through pinholes as the image does not invert when we squint or put our fingers together in front of an eye.
Ah, maybe I’d better just draw a picture…
The normal image on the back of your retina is already inverted. The pinhole won’t change that, so everything looks normal.
I won’t claim to understand the optics behind it (something to do with refraction, right?), but looking through a pinhole definitely improves vision. I have pretty bad eyesight and can’t read the time on my alarm clock without my glasses. But I can form a pinhole with my fingers and voilà! I can see well enough to make out the time.
The same thing happens when you squint: you close down your eyelids enough to form a sort of pinhole.
I have a degree in Optics, friends.
Squinting des NT change the shape o the eye. The pinhole theory is closer to being correct – when you squint you are reducing the effective aperture of the eye, at least in one direction. You can get the ame effect by looking through a small hole in something (like the punchhols in your watchband, for instance), and someone has actualy tried to sell “glasses” that are basically just mounted pinholes.
It’s not really a matter of “stray light”, which is how we usually describe light that rattles around far from the optic axis and doesn’t contribute to forming an image. The problem is that your eye obviously isn’t focusing the light in the right place, and you can reduce the size of the blur by restricting the rays that come in farther off-axis, which are likely to strike the focal plane farther from their ideal point. It also increases your “depth of focus”, meaning that yu can be farther out of focus and still se a relatively sharp imae. The extreme case is an infinitesimally tiny pinhole. This has infinite epth of focus, so the image s always sharp. The problem is that you’ve traded depth of focus for brightness of image. The whole point of the cornea-lens combination of your eye is to collect a lot of light and to image it. Notice that when you squint things grow dimmer – that’s your tradeoff.
You’ll also notice that there are often weird “rays” coming off of bright light sources, and strange “filaments” you didn’t see before. That’s diffraction effects making themselves known. Bt I’m not going to get into THAT now.
ZenBeam
Yes, the eye inverts what it sees, but were used to it and convert it back automatically. Are you saying that a pinhole will not cause a second inversion? Doing ray tracing, that doesn’t seem to be the case. Instead, the image seems to be inverted at the pinhole, then inverted a second time inside the eye.
CalMeacham,
It sounds like you are using pinhole and aperture to describe the same thing. As I understood it, an aperture is a stop that blocks some level of higher order light from entering a lens, while a pinhole is an optic that is meant to pass only zero order light and can be used to resolve an image without the assistance of other optics. Am I in error about the terminology? Admittedly, my optics training was mostly OJT and sitting in on classes; nomenclature isn’t my strongpoint.
cornflakes:
Almost. I’m using “pinhole” to refer to an aperture that is fully “stopped down”. I suppoe you could call it a “zero order” aperture, but I’ve never heard it described that way. The point is that most aberrations increase rapidly with the size of the aperture, so stopping down the apertue (making it smaller) will make your image sharper, in general. Unfortunjately, the illumination also goes up rapidly with increasing aperture, so stopping down throws away a lot of light.
For a pinhole camera you either need a lot of light, or a long exposure.
Yes, looking through a pinhole will not cause a second inversion. I’m not sure how you’re getting a second inversion in your ray tracings, unless you’re projecting the image from the pinhole onto some surface, then looking at that, but this isn’t the same as looking through a pinhole. I just tried poiking a small (0.5 mm) hole in a piece of paper and looked through it. No inversion.
By George, I think he’s got it!
ZenBeam, CalMeacham, of course, you are right. Thinking about it, it strikes me that a lens that follows a pinhole can invert, noninvert or zoom, depending on the distances involved. Blame the fact that we use big lenses at work for this…
So ZenBeam, the .5mm pinhole is giving you a resolution of about .5mm*, reduced by whatever the reduction factor is for your eye… right?
- resolution ~ (pinhole dia)/f1*(f1+f2)
No other lenses involved; f1 and f2 measured from pinhole.
Cornflakes:
Where did your formula come from?
A pinhole with a long separation after it and no lens is the classic case of Fraunhaufer diffraction. An aperture with a lens after it and a screen at the focal point is Fresnel diffraction. Both give you an Airy pattern of a bright central disc surrounded by “haloes”, but neither gives quite the formula you have rendered. A pinhole with no lens after it and a separation that is not really large gives a more complex form, for which you should consult a book on diffraction theory or on the practical aspects of pinhole photography.