The pupil is the adjustable opening in the center of the iris that regulates the amount of light entering the eye. It appears black because most of the light that passes through is absorbed inside the eye.

In bright conditions the iris constricts the pupil, reducing light intensity on the retina and increasing depth of field. In dim conditions the pupil dilates to admit more light. The response is automatic and consensual: light in one eye produces constriction in both. Pupil size is also influenced by cognitive load, emotional arousal, and certain drugs. Clinicians examine pupil reactivity as a quick check of brainstem function.

The pupil’s diameter can range from roughly two to eight millimeters. Extreme dilation or constriction can be produced by medications or neurological conditions. Because the pupil is the aperture of the optical system, its size affects both the brightness and the sharpness of the retinal image.

Every photon that contributes to vision first passes through the pupil.