How The Eye Works as a Camera

eyeTo understand macular degeneration, one must know how the human eye works.The eye can be compared to a camera.

The cornea is the transparent, curved front layer of the eye. The pupil, behind the cornea, is a hole in the colored membrane called the iris. Tiny muscles in the iris change the size of the pupil – like the aperture of a camera – to control the amount of light getting into the eye. There is a small, powerful lens behind the pupil which changes shape based on the pull of muscles in the eye. Like a camera lens, the lens in the eye focuses incoming images from the outside world (incidentally, cataract surgery removes this lens and replaces it with a clear artificial one).

The retina is a thin membrane which covers the inside back of the eye. Like film in a camera, the retina is the light-sensitive surface where images from the outside world come to a focus. The retina receives those images with millions of microscopic photoreceptor cells known as rods and cones. Rods are sensitive to light and dark, shapes and movement. Cones are sensitive to fine detail and to color.

The retina includes two areas called the macula and the fovea. The macula is the name for a tiny oval near the center of the retina, where cones are in high concentration and visual perception is most acute. The macula is responsible for receiving the sharp, straight-ahead images which are used in reading, driving, facial recognition, and so on. It is a hundred times more sensitive to detail than the rest of the retina. The fovea is the center of the macula. It looks like a pit, and has the highest concentration of cones anywhere on the retina.

Under the neural retina is the retinal pigment epithelium (RPE), which supplies nutrients to the photoreceptor cells – the rods and cones. Behind these layers of the retina is the choroid; this is connective tissue and has blood vessels called capillaries. Behind the choroid is the sclera – the supporting structure of the eye.

The retina (including the macula and fovea) sends images collected by the rods and cones as electrical impulses up the optic nerve, which is essentially a cable built of thousands of nerve fibers. It sends these impulses to the processing center of the brain, where they are interpreted as sight.