The pupillary reflex is an automatic response of the eye that controls the size of the pupil in reaction to light. When bright light enters the eye, the pupils constrict to limit the amount of light that reaches the retina, protecting it from damage. Conversely, in low light conditions, the pupils dilate to allow more light in, enhancing vision. This reflex involves the iris, the colored part of the eye, and is controlled by the autonomic nervous system. It is an important function for maintaining optimal vision and is often tested by healthcare professionals during eye examinations.