What is the function of the iris in the human eye?

The iris is a crucial part of the human eye, primarily responsible for controlling the size of the pupil. This function is essential for regulating the amount of light that enters the eye, thereby protecting the retina and enhancing visual clarity under varying lighting conditions. When light is bright, the iris constricts the pupil to limit light entry and prevent damage, while in dim conditions, it dilates the pupil to allow more light in for better vision.

While other components of the eye, such as the cornea and lens, focus light onto the retina, and the optic nerve transmits visual information to the brain, these functions are distinct from that of the iris. Additionally, the eye does have mechanisms to protect it from foreign particles, but this is not the primary role of the iris. Thus, the function of the iris in controlling the size of the pupil is what makes this answer the most accurate in the context of eye anatomy and physiology.