Thursday, January 04, 2007

Why Does Squinting Help?

Via CognitiveDaily , LiveScience has a discussion of how squinting appears to help us see better. The gist of it is, according to the article, that
Squinting reduces the amount of peripheral light coming into the eye so that a greater percentage of light comes from the center of the visual field.
I don't find that explanation particularly illuminating. Why should blocking peripheral light make the image sharper?

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When I have investigated this myself, it seemed clear to me that the key point of squinting is that it blocks all light except for that coming from a tiny hole. If you are nearsighted, then you can verify this for yourself by taking a sheet of paper, and poking a tiny pinhole through it. Images through the pinhole are much sharper. But why is that? I suppose that one could think of the pinhole as somehow like a lens, but it has one big difference from glass lenses: In the case of lenses, a lens that will help nearsightedness will make farsightedness worse, and vice-versa. In contrast, a pinhole seems to help both nearsightedness and farsightedness.

Well, here's my attempt at a simple explanation. Instead of dealing with eyeballs and retinas, I'm going to switch to the problem of how to take a picture. You want a sharp image of a point on the object you are photographing (I've chosen an arrowhead in the pictures below) to appear at a precise spot on the photographic plate or film.

Figure 1 below shows what happens if you just hold up a photographic plate next to the object. Light from your object shows up everywhere on the plate, and so you just get a smear of many, many images.

Figure 2 shows how a lens helps things. Light passing through the lens is bent, so that light rays directed at the top of the photographic plate are bent downwards, and light rays directed at the bottom of the photographic plate are bent upwards. This effect (with the right lens and the right distances between object, lens and plate) causes all the rays from the arrowhead to land at the same spot on the photographic plate. So you get a sharp image.

Figure 3 shows the effects of using a pinhole. Unlike a lens, the pinhole doesn't do any focusing. Instead, it blocks most of the rays coming from the arrowhead. Only those rays pointing in roughly the same direction make it to the photographic plate. So a sharp image is made on the plate.
Note: Figure 3 has one glaring inaccuracy about it. What is it?

Figure 1: Image is blurred on film, because light from arrowhead is spread throughout the photographic plate.

Figure 2: Inserting a lens bends some light rays up and some light rays down, so that light from arrowhead is focused at one spot on the plate.

Figure 3: Inserting a screen with a pinhole removes all images of the arrowhead except those that fall in one small region of the photographic plate.


Blogger RichM said...

I don't think Figure 2 looks so hot either.

Check out these cool pinhole array glasses from New Zealand. I would like to have a pair just so that I could go around looking like some creature with compound eyes. Of course one could go the diy route.

8:38 PM  
Blogger Daryl McCullough said...

Yes, Figure 2 has the same problem. I was only worrying about the location of the arrowhead tip, and wasn't paying attention to the orientation.

I'd like to try the pinhole glasses. Probably on a sunny day at the beach, they would work better than lenses, because they would improve clarity and also cut down on excess sunlight.

On the other hand, the diy aluminum foil glasses look too geeky even for me.

10:39 PM  
Anonymous BASTA! said...

Squinting also helps myopic people to see more clearly because eyelids slightly compress the eyeball along the optical axis of the eye, bringing retinas closer to the focal surface which is normally in front of the retinas in myopic people.

2:24 PM  

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