pigmented colorants: dependence on media and time - Cornell ...

the iris. When this band of muscles contracts, the pupil gets smaller. The second
set of muscles run radially from the edge of the first set of muscles. When this
set contracts, the eye dilates and the gap widens. These muscles determine the
amount of light that reaches the back of the eye.
The size of the pupil is dependent on the availability of light in the surroundings.
The size of the pupil is inversely proportional to the amount of available light.
Hence, your pupil opens more in low-light situations. In young adults, the pupil’s
diameter varies over a range of 4:1, which fluctuates the amount of light from 16:1
[SB02]. To understand why large pupils are not always used, we once again turn
to photography. The aperture, or opening, of a camera restricts the diameter of
the cone of light which reaches the film. A device called the diaphragm controls
the opening and can be modified in a similar manner to the iris. Reducing the
aperture size increases the depth of field, or the range of objects in focus in an
image. While sharp images are desirable, they require small apertures, resulting
in the need for much more light.
A very important optical element in the eye is the crystalline lens, which resides
right behind the iris. The covering of the lens is an elastic capsule, which varies the
optical power of the lens by molding the shape. By shifting one’s focus between
near and far objects, you are consciously adjusting the shape of the lens. This fine
tuning of the focus of image on the back of the eye is know as accommodation.
A condition of the hardening of the fibers in the lens is called sclerosis, which di-
minishes one’s ability to accommodate objects of varying distances. Similar to the
cornea, the lens is also susceptible to transparency issues. Reduced transparency
in the lens is called a cataract.
The innermost of the eye’s three layers, the retina, is where light from the
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