[BAMM-BAMM]

I have been wearing reading glasses for about three years - never had any vision problems, except for standard color blindness.

Some days, it seemed, my eyes were better than other days. But recently, while sitting in the open sun waiting for a car repair, I was able to read everything in my wallet, right down to the tiniest print without glasses. So there must be a connection between the amount of light and the ability to focus, but I've never heard of such a thing.

Any comments?

Thanks

Bryan

[dmatos]

There is, in fact, a relationship between aperture (the size of the hole the light comes through) and focal depth, as any photographer can tell you.

With a large aperture, light rays from a point source can go through any part of that hole. The lens must be perfect over the entire hole, or you will lose focus. With a tiny aperture, the angular separation of the light rays from a point source going through the hole is much smaller, and the lens only has to be good over a smaller area to get a sharp image. Additionally, if the lens is less than perfect, the spot size on the focal plane will be smaller for a smaller aperture, making things clearer.

At the extreme end of this scale is the pinhole camera. The aperture is so small that only one light ray from a point source can go through, and a clear picture will be projected onto the focal plane with no lens whatsoever.

In cameras, having a small aperture means that you can have more things at different distances in focus at the same time. A large aperture gives a narrow depth of field, where only items that are very close to the same distance away from the lens can be in focus at the same time.

With your eye, the lens is faulty. However, when the aperture is very small, it doesn't matter quite so much. In bright sun, your pupil contracts to a very small hole, so your eyesight will be better. You can also try squinting (yes, there is a reason that this works), or peering through a tiny hole made by curling your index finger up.

Of course, the downside to the smaller aperture is that less light gets in. It becomes difficult to collect enough light to see (or take a photograph) in low-light situations.