Astigmatism occurs if the cornea is shaped like a rugby its curvature is not uniform. As a result horizontal lines are focused to a different point from vertical lines.
若角膜呈榄球形状，即其弧度呈不规则，水平光线将聚焦在和垂直光线不同的焦点，这便是散光。
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In astigmatism, the surface of the cornea or lens has a somewhat different curvature in one direction than another.