The amount of light entering the human eye is regulated by the diameter of the pupil. The pupil is the dark spot at the center of a healthy iris that, according to the National Institutes of Health, rests immediately in front of the eye's lens.
The pupil plays much the same role in the vertebrate eye as a shutter does in a camera. When the pupil constricts, the amount of light that is able to pass through to the lens is reduced. An open or dilated pupil is able to admit more light and appears as a larger dark circle in the center of the iris than does the pinpoint of a constricted pupil. This expansion and contraction is controlled by tiny muscles in the iris, which is the colored part of the eye. About.com lists normal pupil size in humans as ranging between 2 mm and 5 mm, depending on external light conditions, with young people typically having somewhat larger pupils than adults.
The appearance and behavior of the pupil can be useful to doctors, according to About.com. Unequal pupil diameter naturally occurs among some people, but unequal or abnormal pupils can be a sign of head trauma, cancer and certain diseases such as syphilis.