human retina
2002-09-07 by MARK MAIO
> > > Also, humans have variable resolution in their > > > sight... > > > > No, they do not. The maximum resolution is set by physiology, as > > described > > above. Some people have less; but nobody has more. > > They do not what? It's a physical fact that your eye has higher resolution > in the center, and decreased resolution around the periphery. > My two cents regarding vision. The normal length human retina has only one small area, called the fovea, where light is in focus in all of us who do not require contacts or glasses. Eyes shorter or longer in length require + or - lenses to focus the image at the correct distance. While the majority of the remainder of the retina is curved and on the same plane of focus, the center of the fovea is slightly indented and at a different focal plane. This area is where we do our critical seeing and where the image is sharp. You are reading these words using the fovea of your eye. Everything else in our visual pathway is considered peripheral vision and while we really don't notice it, is out of focus. We have "learned" to "see" as if everything is in focus. Other major physical components or problems with our eye which affect our resolution of detail: 1. The cornea, which is like the glass crystal on our watch, is part of our optical system. Any defect here (most common is astigmatism), affects the resolution of detail on the retina. 2. The lens. Most common is our losing the ability to accommodate (which is to say "focus"). Most of us notice this around the age of 40 when the "zonules" (which attach to the lens and act as the focusing mechanism), lose their ability to shape our lens so it can focus at a close distance. Once our arms get too short (as in I can't hold the book far enough away to see it in focus), we need to get reading glasses. The other common age related visual problem is cataracts. The formation of most cataracts is due to us living longer than we were designed to. As we get older, our lens starts to harden from the inside center, out. Think of it like the the optical center of your photographic lens taking on the properties of frosted glass. The cataract starts as a mild defect and usually progresses to the point where it (as in your lens), has to be removed and replaced with a plastic lens. As the cataract is forming over a period of years, we not only slowly, and without really noticing, lose the ability to distinguish fine detail, but the cataract also turns the lens yellow, affecting how we see color. One way to slow this process down is to always wear UV blocking sunglasses outside. To qualify the above information, although my formal education is in photography, my knowledge of the eye has come from working since 1978, in the field of ophthalmic photography (we photograph the eye, inside and out, for diagnosis, treatment and research), nine years of which I held the position of assistant professor in the Medical School of the State University of New York. And before anyone asks if they should have Lasik surgery so they can stop wearing glasses, I'll give you the answer I tell my relatives, which is I don't know one ophthalmologist (and I know hundreds), who has had the procedure done. Mark Maio [Non-text portions of this message have been removed]