The colored ring of the eye that controls how much light enters through the pupil is the iris. It surrounds the pupil and changes the pupil’s diameter, acting much like the adjustable diaphragm in a camera.
The iris contains two main groups of smooth muscle. The circular sphincter pupillae muscle constricts the pupil in bright conditions, while the radial dilator pupillae muscles enlarge it in dim conditions. This automatic adjustment is part of the pupillary light reflex and helps regulate the amount of light reaching the retina.
The pupil itself is not a colored structure or a muscle; it is the opening at the center of the iris. It appears black because most incoming light is absorbed inside the eye. The retina, at the back of the eye, receives the focused image and converts light into nerve signals.
Several neighboring parts are easy to confuse. The sclera is the white outer coat, the cornea is the transparent front surface, and the iris is the visible colored ring behind the cornea. Eye color is determined mainly by pigment and light scattering within the iris.