The pupil is the opening in the center of the iris (the structure that gives our eyes their color). The function of the pupil is to allow light to enter the eye so it can be focused on the retina to begin the process of sight.
Together, the iris and pupil control how much light enters the eye. Using the analogy of a camera, the pupil is the aperture of the eye and the iris is the diaphragm that controls the size of the aperture.
The size of the pupil is controlled by muscles within the iris — one muscle constricts the pupil opening (makes it smaller), and another iris muscle dilates the pupil (makes it larger). This dynamic process of muscle action within the iris controls how much light enters the eye through the pupil.
In low-light conditions, the pupil dilates so more light can reach the retina to improve night vision. In bright conditions, the pupil constricts to limit how much light enters the eye (too much light can cause glare and discomfort, and it may even damage the lens and retina).
The size of the pupil varies from person to person. Some people have large pupils, and some people have small pupils. Also, pupil size changes with age — children and young adults tend to have large pupils, and seniors usually have small pupils.
Generally, normal pupil size in adults ranges from 2 to 4 millimeters (mm) in diameter in bright light to 4 to 8 mm in the dark.
In addition to being affected by light, both pupils normally constrict when you focus on a near object. This is called the accommodative pupillary response.
Conditions That Affect The Pupil
A number of conditions can affect the size, shape and/or function of the pupil of the eye. These include:
Adie's tonic pupil: This is a pupil that has nearly no reaction to light (direct or consensual) and there is a delayed reaction to accommodation. Adie's tonic pupil (also called Adie's pupil, tonic pupil, or Adie's syndrome) usually affects only one eye, with the affected pupil being larger than the pupil of the unaffected eye.
Argyll Robertson pupil: This is a pupil that is not reactive to light (direct or consensual), but reaction to accommodation is normal. Argyll Robertson pupil usually affects both eyes, causing smaller-than-normal pupils that do not react to light.
Marcus Gunn pupil: Also called relative afferent pupillary defect (RAPD) or afferent pupillary defect, this is an abnormal result of the swinging-flashlight test where the patient's pupils constrict less (therefore appearing to dilate) when the light is swung from the unaffected eye to the affected eye.