Astigmatism causes light entering the eye at different axes to be focused different amounts. For example, light entering vertically (from 12 o'clock to 6 o'clock) may be focused more than light entering horizontally (from 9 o'clock to 3 o'clock). In an eye without astigmatism, light is focused the same amount in each axis.
The net result of astigmatism is blurred vision. Often letters appear slanted or with "tails" coming off of them. Sometimes the affected eye sees double.
When LASIK is performed for astigmatism, the cornea is reshaped by the laser to allow proper focusing, regardless of the axis light enters the eye. In practical terms, astigmatism is treated similar to nearsightedness and farsightedness, but with different amounts of treatment to each axis.
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