The rest comes from the cornea, which acts like a second lens to help focus light onto the retina. If you're nearsighted, or myopic, your eye produces clear images of nearby objects or people. If you're farsighted, or hyperopic, on the other hand, the focal point for distant objects is fine, while the one for close sources actually falls behind the retina. In this case, the cornea is too flat relative to the length, or vice versa. Astigmatism occurs because the cornea's curvature is not uniform, making both distant and nearby objects blurry.
Age adds another complication.
Most people, as they get older, need reading glasses or bifocals for close work. Attempts to change the way the cornea focuses light by surgically altering its surface began as early as the 1950s. By the 1970s Soviet doctors routinely used scalpels to reshape the corneas of nearsighted patients in an operation called radial keratotomy. But the surgery, involving a spokelike ring of incisions, never really caught on in the U.S., because the results were so difficult to predict and the healing process was often slow and painful. Enter the excimer laser.