Difference Between Farsighted And Nearsighted Ans Astigmatism
A farsighted eye can give clear images of distant things but not of those that are fairly near. The reason may be poor power of accommodation. Or it may be an eyeball that is too short, front to back, for a sharp image to form on the retina. In either case, the condition is helped by using eyeglasses with converging lenses, to help bring the rays together properly.
Some eyes are nearsighted. They cannot form sharp images of objects that are more than a few centimeters away. Usually this is because the eyeball is too long, front to back. This condition is helped by eyeglasses with diverging lenses. These lenses spread the rays slightly, so that a sharp image results.
Another common defect is called astigmatism. In this case the front surface of the eyeball is not of the same roundness in all directions. If you have astigmatism, you will find that the various sets of lines will not appear equally sharp. One group will be clearer than all the others. And the group that crosses it squarely will seem fuzzier than the rest. Astigmatism can be corrected by using eyeglass lenses that are curved more in one direction than in another.
An eye may need correction for nearsightedness or farsightedness and for astigmatism at the same time. The two lens shapes that are needed can be ground into the same piece of glass.
Small, thin lenses that rest on the eyeball itself have become very popular. These are called contact lenses. They float on the layer of moisture (tears) that covers the eyeball.