Optical discrepancies between the various components of the eye cause a majority of the human population to have some form of refractive error. People who are unable to bring distant objects into clear focus are nearsighted, or myopic. Myopia can be caused by the front of the cornea being too curved or the eyeball being too long. In either case, with the lens as flat as it can be, the image of distant objects focuses in front of, rather than on, the retina.

Neuroscience, Sinaur Associates (QP355.2.N487 1997)