Astigmatism is an imperfection in the curvature of the cornea, which is the clear, round dome covering the eyes iris and pupil. Normally, the cornea and lens are smooth and curved equally in all directions, helping to focus light rays sharply onto the retina at the back of your eye. However, if the cornea or lens is not smooth and evenly curved, light rays are not refracted properly. This is what happens in astigmatism. It can be corrected by using cylindrical lenses.