dog eye

Like tourists who assume everyone speaks English, or should, it is second nature to us to think that the world looks pretty much the same to all creatures, great and small, including our dogs. For example, we rarely give much thought to the optical processes that turn light into vision; we assume that our visual version of reality is reality.

Even those of us who wear glasses fall into this way of thinking. Glasses bring things back into focus so they once again look like they are. If those people who run around staging role-playing seminars on multiculturalism for business executives were to do the same for multi-species, I would suggest as the first group exercise they get everyone down on the floor with their eyeballs about six inches off the ground. Simply by virtue of visual perspective, the world looks very different to a Chihuahua.

Dogs also differ from humans in their ability to focus on near objects, to perceive and distinguish detail, and to see contrasts between light and dark. Some of these differences are relatively minor, but some must result in a highly altered version of reality. The most remarkable feature of the human eye is its extraordinary power of "accommodation." The lens in a normal eye, when relaxed, is of just the right thickness and curvature to bend incoming light rays from a far distance (equivalent to the setting of "infinity" on a camera lens) so that they converge in sharp focus upon the retina at the back of the eye. If the lens were incapable of adjustment, the light rays from close objects would end up converging at an imaginary point well behind the retina; the result would be a grossly blurred image striking the light-sensitive cells of the retina. But by squeezing the lens with muscles that are under unconscious control, we can make the lens thicker and alter its curvature, bringing close objects into proper focus. The greater the squeeze, the closer to our face is the focus.

In young children, the eye's lens is capable of adjusting by as much as 14 diopters, an optical unit used in describing the power of lenses (and in prescribing eyeglasses). That degree of accommodation corresponds to being able to focus on everything from infinity to an object less than three inches away. By way of comparison, eyeglasses with a power of 14 diopters would look like the proverbial Coke bottle bottoms. (Most glasses for correcting nearsightedness in humans run about 1 to 5 diopters.)

Dogs have a much more limited power of accommodation, generally not more than 2 or 3 diopters, which means they can focus on close objects only if they are no nearer than a foot or two. Anything closer than that will unavoidably be a blur. That may well explain why dogs generally try to sniff or touch objects at close range: they simply cannot see them very well. If the relaxed lens normally brings a distant object's image into focus behind the retina, the result is hyperopia or farsightedness.


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