The human eye works like a camera, with a lens, diaphragm, and film. The cornea and the crystalline form the lens. In between is the iris, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. The cornea and the crystalline lens focus the image in a sharp way on the retina, which serves as a film that covers the back of the eye. The visual stimuli on the retina are transformed into electrical pulses and reach the brain through the optic nerve. The light rays are focalized on the retina and allow for optimal vision. If there is a defect in this optical system (myopia, hypermetropia, astigmatism or presbyopia), images will be blurred upon reaching the retina.