Astigmatism

An optical imperfection due to the cornea or lens of the eye being ‘out of round’ or not uniformly evenly curved. Example: the side of a donut is a ‘toric’ or astigmatic surface as the horizontal curve is different from the vertical curve. The difference between the two curvature radii equals the amount of astigmatism. Astigmatism is corrected by lenses ground in a cylindrical shape (as compared to concave or convex lenses which correct near- and farsightedness, respectively). For that reason, the astigmatism component of an optical lens is sometimes referred to as the ‘cylinder’.