Some eyes focus, or refract, the light too much so the images of distant objects are formed in front of the retina, and the image on the retina is blurred. This condition is called nearsightedness or myopia. Myopia usually starts in childhood and gets progressively worse through adolescence. It usually stops changing by the late teens, but it can sometimes continue to get worse into the middle 20s.
Nearsightedness can be corrected by any method that reduces the total refractive power of the eye. Eyeglasses and contact lenses do this by putting in front of the eye negative lenses that are thicker at the edge than in the center. Refractive surgery corrects nearsightedness by flattening the central part of the cornea to reduce the total refractive power of the eye.