The part of the human eye that performs a function similar to the diaphragm in a camera lens is the Iris. Just like the diaphragm in a camera lens controls the amount of light entering the camera, the iris in the eye controls the amount of light that enters the eye by adjusting the size of the pupil. It can contract or expand to regulate the size of the pupil in response to varying light conditions, thus controlling the amount of light that reaches the retina at the back of the eye.