The eye transforms light beams into electric impulses. The brain translates these impulses so we can see. In order to get a sharp image the light beams must be bent in order to converge perfectly on the retina in the back of the eye.
First the light beams are bent by the surface of the cornea and finally by the natural lens. The cornea is transparent and has a "ball" shape. The natural lens, located behind the iris, is flexible and can change its shape (accommodate). But the accommodative property of the lens stops at the age of 40 years. The lens becomes less flexible which leads to presbyopia.
