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The World
According To Your Dog's Eyes
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.
There are more
information articles on all aspects of basics dog training, dog health
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The World
According To Your Dog's Eyes
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