Eye-Structure Abnormalities
The process described above works best if the
eye is a perfect sphere and the cornea is smooth and
rounded. Often this isn't the case. The eye might be
shorter or longer from front to back than it is from
top to bottom, or the cornea's curvature might be
slightly flattened.
Farsightedness and Nearsightedness
If your own eyes are shorter than normal from
front to back, then you have hyperopia, or farsightedness,
and you see distant objects more clearly than
near ones. If the opposite is true-your eyes are too
long from front to back-you have myopia, or
nearsightedness, and you have trouble seeing distant
objects.
Astigmatism
The cornea's curvature should be rounded, like
the side of a basketball. If, instead, it's shaped more
like the side of a football, it will produce two focal
points instead of just one. This condition is called
15
The Human Eye and How It Works
Also known as being nearsighted, myopia causes distant
images to appear blurry because images focus in front of the
retina rather than on it.
Myopia
astigmatism and it causes blur and distortion,
especially up close.
After passing through the cornea, light is bathed
in a thin layer of liquid, the aqueous humor, before
it reaches the iris. The iris contracts or expands
(dilates) around the pupil to regulate the amount of
light allowed into the eye's interior. Sophisticated
cameras have light-regulating mechanisms that do
much the same thing.
When you walk out of a dark movie theater into
bright daylight, you've probably experienced that
"blinding" sensation before the iris has time to
contract, adjusting to the difference in light.
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