People have been fascinated with
vision since the days of the ancient
Greeks, who thought sight involved
rays emanating from the eye to illuminate
surrounding objects. It wasn’t
until fifteen hundred years later
that the Arab mathematician and
scholar Al-Hazen (965-1040 A.D.)
described how lenses can focus and magnify images.
Today, we know that natural selection has produced at least ten
types of vision systems. Eyes for different organisms are adapted
for seeing in the day or night, short or long distances, with wide
or narrow fields of view.
Still, all of these systems capture light and use it to produce
some sort of picture in the brain representing the surrounding
environment. Often they are more efficient, more powerful,
simpler and more elegant than their manmade counterparts.
There are two main types of vision systems: camera-type eyes,
which use a single lens to focus images onto a retina, and compound
eyes which have multiple lenses — sometimes tens of
thousands of them.
Camera-type eyes use a variety of ways to focus the lens so they
can see both near and far. Humans and birds have specialized
muscles that change the lens’ curvature. In whales, fluid behind
the lens fills and empties to move the lens closer or farther from
the retina, while in amphibians a muscle moves the lens back
and forth. Compound eyes can only see nearby objects clearly.
The single lens in the octopus eye has layers like an onion, each
with slightly different optical properties, to help focus the light,
even with a wide field of view.