The primary function of the iris is to:

The primary function of the iris is to regulate the amount of light entering the eye. The iris is the colored part of the eye surrounding the pupil, and it consists of muscles that control the size of the pupil. When light levels are high, the iris constricts the pupil to decrease light entry, preventing damage to the retina and improving visual clarity. In contrast, in low-light conditions, the iris relaxes, allowing the pupil to dilate and admit more light to enhance visibility.

Understanding this mechanism highlights the importance of the iris in maintaining optimal vision under varying lighting conditions. The iris plays a crucial role in protecting the sensitive retinal cells from excessive brightness while ensuring that sufficient light reaches them for proper image formation. This regulation of light entry is vital for adapting to different environments, thereby emphasizing the iris's role as a key component in the visual system.