Astigmatism occurs when the cornea is oblong—in other words, it is shaped more like a football than a basketball. The oblong shape causes light rays to focus in two places on the retina, causing blurred vision.

Astigmatism may also accompany nearsightedness or farsightedness, and can get worse with age. A small amount of astigmatism may cause only slightly blurred vision. Sometimes uncorrected astigmatism can lead to headaches or eyestrain.