The iris controls how much light enters the human eye by changing the size of the pupil.
The iris is the colored, contractile diaphragm located between the cornea and the lens. Circular sphincter pupillae muscles constrict the pupil in bright conditions, while radial dilator pupillae muscles enlarge it in dim conditions. The pupil itself is an opening, not a separate tissue that contracts.
Light passes through the cornea, pupil, and lens before reaching the retina. The lens changes shape to focus images at different distances, while the retina contains photoreceptors that convert light into neural signals.
Pupil size is also influenced by the autonomic nervous system. Parasympathetic activity generally produces constriction, and sympathetic activity generally produces dilation. The visible eye color comes mainly from pigments and the structure of the iris, especially its stroma.