To see clearly, the cornea and the lens must bend — or refract — light rays so they focus on the retina — a layer of light-sensing cells that line the back of the eye. The retina converts the light rays into impulses that are sent to the brain, where they are recognized as images. If the light rays don't focus on the retina, the image you see is blurry. This is called a refractive error. Glasses, contacts and refractive surgery attempt to reduce these errors by making light rays focus on the retina. Refractive errors are caused by an imperfectly shaped eyeball, cornea or lens, and are of three basic types: * myopia — nearsightedness; only nearby objects are clear. * hyperopia — farsightedness; only objects far away are clear. * astigmatism — images are blurred at a distance and near. There's also presbyopia — "aging eye." The condition usually occurs between ages 40 and 50, and can be corrected with bifocals or reading glasses.