The iris is the coloured part of the eye. A circular muscle in the iris controls the amount of light entering the eye through the pupil, the black area in the centre of the iris.

The lens is located behind the iris and is normally clear. Light passes through the pupil to the lens. Small muscles attached to the lens can change its shape. Tightening or relaxing these muscles causes the lens to change shape, allowing the eyes to focus on near or far objects (accommodation).