The optical stage of vision is associated with the front of the eye, the first part of the visual pathway.
Adjusting the amount of light: the iris
The pupil is the opening that lets light enter the eye and ultimately reach the retina. The pupil, the colored part of the eye, appears black because of the layer of black pigmented cells that line the back of the eye and absorb the light.
The diameter of the pupil is controlled by the iris, an adjustable aperture whose contraction lets the eye adapt continuously to changing light conditions. Like the diaphragm of a camera lens, the eye’s iris opens and closes around the pupil to adjust how much light is let in. The darker the lighting you are in, the wider the iris opens.
When the lighting is bright, the iris constricts and the aperture (the pupil) though which the light enters the eye becomes smaller. This prevents the light sensitive receptors (rods and cones) in the retina from becoming saturated with too much light.