What is Astigmation

Astigmatism

 A property of a lens or mirror system in which a surface is not that of a perfect sphere or parabola, but has a greater degree of curvature in one meridian than in the meridian at right angles to that one. Such a toric surface produces two foci—one for rays in the plane parallel to that of greatest curvature and one for rays in the plane at right angles. Only one of these sets of rays can be focused at one time. The cornea of the eye is commonly astigmatic, usually being most steeply curved from top to bottom and least steeply curved from side to side. Ocular astigmatism is corrected by eyeglasses having cylindrical lenses set at the appropriate axis so that the steeper corneal curve is matched by the less steep eyeglass lens curve.