What is the function of the iris in the eye?

According to the University of Michigan Kellogg Eye Center, the function of the iris in the eye is to control the amount of light that reaches the retina. It does this by changing the size of the pupil, the transparent window that allows light into the eye's interior.

The University of Michigan Kellogg Eye Center explains that the iris automatically grows and shrinks the pupil in response to light levels, with bright light causing a small pupil and darkness causing a large pupil. This is important for the light-sensing cells in the retina, which require enough light to activate but not so much as to overexpose or even damage them.