Vision depends on light, either from the sun or from an artificial source such as a light bulb. Light reflects from objects and enters your eyes through a clear covering, the cornea, over the front of the eye. The cornea does most of the focusing of light onto the retina. Behind the cornea is the lens which does some of the focusing and enables the eye to vary its focus. Thus an image is formed on the back of the eye, on the retina. The most light-sensitive point of the retina, near its center, where sharpest vision is achieved, is called the macula. The retina turns the picture into tiny electric impulses that are sent by way of the optic nerve to the brain, where seeing actually occurs. Shows where the various parts of the eye are located.

Central vision is what you see when you look straight at an object. Peripheral (side) vision is what you see of the area surrounding the object at which you are looking. In central vision, the picture is focused on the macula. You look at a word on a page with your central vision, and with your peripheral vision you can tell if the word is at the beginning or end of the line or at the top or bottom of the page. Even
while you are reading with central vision, you can see someone enter the room with your peripheral vision. Peripheral vision also lets you walk or drive without bumping into or stumbling over objects. Central vision sometimes is called “seeing or reading” vision and peripheral vision is called “travelling” vision.If your eyes are normal, you can find out what loss of central vision is like by closing one eye and holding a penny directly in front of the other eye and trying to look “through” the penny at a word or at a distant sign. You cannot see straight ahead, but you can see the surrounding area. You can find out what loss of peripheral vision is like by closing one eye and then looking through a long tube of newspaper with the other eye. Under this circumstance, you can see straight ahead but cannot see the surrounding area.

Central vision is measured by looking from a distance of 20 feet at a chart printed with numbers or letters of different sizes. If you can read the small letters near the bottom of the chart, your central vision is said to be 20/20, meaning that you can see at 20 feet what a normal eye sees at 20 feet. If you are 20 feet away from the chart and you can read only the top line, your central vision is much poorer and is 20/200. In this case you are 20 feet away from the chart and are able to read only
those numbers which a normal eye can read at 200 feet from the chart. In between these levels there are various gradations of vision, corresponding to visual acuity of 20/30, 20/50, etc., following the same principle. Peripheral vision is measured by focusing your eyes at a spot straight ahead and
then telling the examiner when you see a light or object which is slowly brought from above or below, from left or right, into your field of vision. Normal peripheral vision (called the visual field) for one eye is approximately 150 degrees from side to side, and for both eyes is approximately 180 degrees. Depending upon the visual problem, all or only part of the visual field may be affected. The fewer
 the degrees of peripheral vision, the greater the loss of vision.