Farsightedness Nearsightedness Astigmatism Focusing an image sharply on the retina (the light-sensitive layer at the back of the eye) depends mainly on the lens, the size of the eyeball, and the cornea. Muscles around the lens contract or relax to change the shape of the lens (and therefore its focusing power) in order to adjust focus between near and far objects.
In farsightedness (hyperopia), the eyeball is too short relative to the focusing power of the lens, making distant objects clear but nearby ones blurred. In nearsightedness (myopia), the eyeball is too long relative to the focusing power of the lens, so distant objects appear blurred but nearby objects can be seen clearly. In astigmatism, the shape of the lens or cornea is irregular.
As a result, all of the light rays entering the eye cannot be focused on the retina, causing vision to be distorted or blurred.
All three of these conditions can be corrected by glasses, by contact lenses, or by laser surgery to reshape the cornea