In order to see clearly, the eye must be able to focus light into a single plane onto the back of the eye at the retina. The word astigmatism comes from the Greek "a" meaning "without" and "stigma" meaning "spot." In astigmatism, a point (or spot) of light is focused at two different planes, causing blurred vision. An optical system (or eye) without astigmatism has only one focus for all rays of light. An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. For example, if an optical system with astigmatism is used to form an image of a plus sign, the vertical and horizontal lines will never be in focus at the same time, since they are in sharp focus at two distinctly different distances. In an eye without astigmatism, the surface of the cornea is shaped like a Ping-Pong ball, where all the curves are the same. This is called a spherical surface. In an eye with astigmatism, the surface of the cornea is shaped more like a football, where there are two different surface curves located 90 degrees apart. This is called a toric surface.
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