By understanding this basic eye anatomy we can begin to understand the ways in which the light signal can be degraded or distorted, resulting in poor vision.
The entry point for light is the cornea, the transparent tissue in the very front of the eye that functions as the window through which all light has to pass on its way to forming a visual perception. Next, the light signal encounters the lens, which focuses it finely on the third structure of the eye: the retina. The retina is analogous to the film in the back of the camera. It is on the retina that the light is converted into a neural signal that is ultimately interpreted by the brain as an image. Finally, the optic nerve, which carries these signals to the brain, functions like a cable that connects the video camera to the television screen. The deterioration of our vision with age is invariably the result of a problem with one of these four structures