Normally, light rays are bent (refracted) by the cornea and lens and focus directly on the retina, the nerve layer at the back of the eye, which results in clear vision.
In most cases, refractive errors represent a natural variation from normal vision and are not considered a disease. Nearsightedness, farsightedness, astigmatism, and presbyopia are common refractive errors.
Vision usually can be corrected with glasses or contact lenses or, in some cases, surgery.
WebMD Medical Reference from Healthwise