The eye is an optical system containing the cornea and a deformable lens, which project the image of the outside world onto the retina, as well as the iris which broadly regulates the amount of the incident light by adjusting the pupil opening. In the retina, the incoming photostimuli are converted into neuron impulses by receptor cells. The eye has two systems of photoreceptors: the rod cells and the cone cells. The rods are distributed relatively uniformly across the retina; they are highly sensitive to light and enable wide-angle vision under low illuminances (scotopic vision). Their visual acuity is low, however, and colours are not perceived. The cones, on the other hand, are predominantly concentrated in the fovea, a small depression in the retina located on the optical axis or visual axis. The cones enable sharp, coloured vision within a limited angle of vision, but require high illuminances (photopic vision).