Which part of the eye controls the amount of light that enters?

The iris is the part of the eye that controls the amount of light that enters. It is a thin circular structure that contains muscles which adjust the size of the pupil, the opening in the center of the iris. When the surrounding light increases, the iris constricts the pupil to allow in less light, protecting the retina from damage and helping to improve clarity of vision. Conversely, in low lighting conditions, the iris dilates the pupil to let in more light to enhance visibility.

While the pupil is the actual opening that changes size, it is the iris that actively regulates this change. The lens focuses the incoming light onto the retina but does not control the light's quantity. The cornea helps in bending light and provides most of the eye's optical power, but it does not regulate light entry. Understanding the distinct roles of these structures clarifies the precise function of the iris in light control.