The analogy itself is a
good one up to a point: The
image of a scene focused on the
retina resembles that focused
on the film in a camera or, for
that matter, a faithful
rendering of the scene by an
artist. The eye has a lens just as
a camera does, and as in a
camera, the lens changes its
focus. The eye has an aperture
(the pupillary opening) that
varies in diameter to admit
more or less light, as does a
camera. The image formed on
the retina is upside down and
reversed left to right, just as is
the image that appears on
photographic film.