Pupil (eye) The pupil is the opening in the centre of the eye. Light enters through the pupil and goes through the lens, which focuses the image on the retina. The size of the pupil is controlled by muscles. There is a circular group, which squeezes the iris smaller, and another group which pulls the iris wider. When more light is needed, the pupil is made larger. In brighter light, the pupil is made smaller.