(a) When a person can see near objects but is unable to see distant objects clearly, the defect is known as near sightedness. This defect is due to either (i) cornea having excessive curvature or
(ii) elongation of the eyeball. As a result the maximum focal length of eye lens becomes less than the distance from the lens to the retina. It can be corrected by using a concave lens.
b) When a person is unable to see equally in all directions, then the defect is called astigmatism. The eye lens develops different curvatures along different planes.
This vision is corrected by using cylindrical lens.