The human eye attains its adult size and shape when a person reaches his or her twenties. At that point, nearsightedness, astigmatism, and farsightedness usually stabilize.
Nearsightedness, known scientifically as myopia, means that a person sees nearby objects more clearly than distant objects. It does not mean that a myopic person has excellent near vision and poor distance vision. Extremely nearsighted people do not see anything clearly. In the normal eye, parallel rays of light passing through the cornea bend slightly and then bend again as they penetrate the lens to focus on a small, single spot on the retina. In the nearsighted eye, a steeply curved cornea or an elongated eyeball, or both, focus rays of light before they reach the retina. From this focal point the light rays begin to diverge. By the time light reaches the retina, the image is blurred.
Farsightedness, known as hyperopia, is the opposite of myopia. It is not necessarily the ability to see objects clearly at a distance and poorly close up, although this can be the case. In the farsighted eye, rays of light streaming through the cornea and lens reach the retina before they come into focus, creating a blurred image. In a sense, the eyeball is too short for the refractive power of its own optical system. Sometimes, mildly farsighted people do not discover this refractive error until middle age, when they begin to have trouble seeing clearly at close distances. This explains the common need for reading eyeglasses among our middle-aged population.
An astigmatic eye results from an imperfectly shaped cornea. The cornea of the astigmatic eye might be poorly curved from top to bottom or from side to side. This error of design gives the cornea different refractory powers across its length and width and distorts sight. Some light rays focus in front of the retina and others, behind it. As a result, the retina never receives a single, sharp image.
Presbyopia is an optical flaw that eventually affects almost everyone. As the eyes age, their powers of accommodation decline. It becomes increasingly difficult for the eye to bring nearby objects into focus. An individual with presbyopia can also be farsighted, nearsighted, or astigmatic at the same time. If this is the case, bifocals can give the eyes extra help with nearby objects, no matter what other refractive errors the eyes may have.
