The muscular shutter at the entrance to the eye controls the amount of light enters in to it. If the light is strong, the muscles automatically close the Iris shutter so that the eye is not overwhelmed. Under bright light the eye may close to such an extent that the vision is interrupted. The Iris opens to its fullest extent when light is dim. The movement of Iris is coordinated with movements of lens. When the eye is subject to constant changes from light to dark and vice versa, the muscles concerned become tired causing temporary disability called eyestrain.