This excerpt taken from the EYE 10-K filed Mar 14, 2006.
Refractive Disorders. Refractive disorders, such as myopia, hyperopia, astigmatism and presbyopia, occur when the lens system is unable to properly focus images on the retina. For example, with myopia (nearsightedness), light rays focus in front of the retina because the curvature of the cornea is too steep. With hyperopia (farsightedness), light rays focus behind the retina because the curvature of the cornea is too flat. Astigmatism makes it difficult for a person to focus on any object because the otherwise uniform curvature of the cornea or lens is somehow disrupted or becomes uneven. Presbyopia is the progressive loss of flexibility of the lens and its ability to change shape to focus on near or far objects, and is presumably caused by aging of the eyes lens and the muscles that control the shape of the lens.