(A, B) A concave (minus) lens is used to correct myopia, in which parallel rays are focused anterior to the macula. (C, D) A convex (plus) lens is used to correct hyperopia, in which parallel rays are focused posterior to the macula. (E, F) A cylindrical (or spherocylindrical) lens is used to correct astigmatism, in which parallel rays are not focused uniformly in all meridians.