Myopia (Nearsightedness)
Also known as nearsightedness, myopia is a condition in which you can see nearby objects well, but ob jects at a distance appear blurred. This happens when light bouncing off a faraway image enters the eye through the cornea and comes to a point of focus too soon, be fore it reaches the retina. Myopia may be due to a cornea that has too much curvature, which causes the light to "overbend" and focus in front of the retina. Myopia also occurs when the eyeball is too long-the retinal wall is too far back for the combined focusing power of the cornea and lens.
Hyperopia (Farsightedness)
People with hyperopia, or farsightedness, see distant objects more clearly than nearby objects when they are young but may have difficulty with both as they get older. In hyper opia, the light rays coming into the cornea are not bent sharply enough and are focused be hind, rather than on, the retina. The result is a blurred image. This usually happens in people whose eyeballs are too short from front to back or whose focusing mus cles around the lens are too weak. Another cause of hyperopia, though rare, is a cornea that is not curved enough.
Because muscles are more elastic in youth, younger people who are mildly hyperopic can actually compensate for it by using the focusing muscles around the lens to fine-tune the focus by bending light more steeply. This action brings the point of focus forward toward the retina, allowing them to see more clearly. However, be cause the muscles weaken and the lens be comes less pliable as we age, these individuals eventually lose that ability and may no longer see well at a distance or close up. After age forty, they may be completely dependent on eyeglasses or contact lenses for both close and distant vision.
Astigmatism
Many individuals with myopia or hyperopia also have some degree of astigmatism. People with significant astigmatism experience blurred or distorted vision with all objects, whether near or far. Astigmatism means that your cornea, in stead of being spherical like the side of a basketball, is slightly oval, shaped more like the side of a football. Your cornea is more curved in one direction than the other. As a result, light rays entering the eye from different points on the cornea's surface are bent irregularly and are focused at several different points, rather than meeting at just one focal point. Almost everyone has a small degree of astigmatism.
Presbyopia
Farsightedness is often confused with presbyopia, which literally means "old eyes." Presbyopia is the age-dependent need for reading glasses or bifocals. After age forty, and in most people by age forty-five, the ability to focus on an object close up, such as a restaurant menu, becomes more difficult. This happens to everyone. It is due to a loss of flexibility in the lens and a weakening in the muscles that enable the lens to flex and fine-tune the focus. Presbyopia typically continues to worsen until age sixty-five. When this occurs, people who already wear eyeglasses may need bifocals, and those who have never worn eyeglasses may require reading glasses.
Low-Order Aberrations