Astigmatism

Astigmatism exists when the radius of curvature is not equal at all places on a curved optical surface. On a sphere, the curvature is identical at all points, and there is no astigmatism.

One example of an astigmatic surface is the side of a donut: The horizontal and vertical curves are different. If a lens were shaped like this, light focused by the horizontal curve would not sharpen at the same point as light focused by the vertical curve. The difference between the two curvatures equals the amount of astigmatism in the lens.

Myopia correcting lenses are spherical and concave. Hyperopia correcting lenses are spherical and convex. Astigmatism correcting lenses are cylindrical both in shape and in the effect of their optical correction. Astigmatism is corrected with the excimer laser by effecting a toric or cylinder-shaped treatment, as opposed to a convex or concave treatment.