The iris has two muscles: The dilator muscle makes the iris smaller and therefore the pupil larger, allowing more light into the eye; the sphincter muscle makes the iris larger and the pupil smaller, allowing less light into the eye. Pupil size can change from 2 millimeters to 8 millimeters. This means that by changing the size of the pupil, the eye can change the amount of light that enters it by 30 times.
