Nearly all refractive errors can be corrected with spectacles or contact lenses, or by surgically reshaping cornea
Myopia is corrected with a spherical concave lens that moves the focus backwards in the eye so that it falls onto the retina
Hyperopia is corrected with a spherical convex lens that moves the focus forward onto the retina
Astigmatism is corrected with a cylindrical lens, which has more focusing power in one axis than another
Presbyopia does not appear until after age 40; before then, the lens is flexible enough so that contraction of the ciliary muscle allows it to assume a convex front surface and focus objects viewed at reading distance; after age 40, the lens proteins gradually degenerate, the lens stiffens, and will not round up to allow focusing of near objects on the retina
Presbyopia is corrected by placing a convex lens in front of the eye. Because it is used only for seeing near objects, it is prescribed either as a half-glass or as the bottom part of a bifocal if the patient needs a correction for distance viewing
