Astigmatism
When you squeeze a tennis ball, the curvature becomes greater in one dimension, and is less in the other dimension. Astigmatism occurs when this difference in curvature occurs in the cornea.
Astigmatism blurs vision because the eye focuses at two different distances at the same time. Because the eye focuses simultaneously at two different distances, objects are never completely in focus. Instead, they are blurred and distorted.
A more technical explanation of astigmatism can be found here.
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When you get your eyes tested for glasses, you are able to detect 1/4 (0.25) Diopter of astigmatism. Many people can detect even less. Small amounts of astigmatism, up to 0.50 Diopter, will not bother you during your normal activities. Vision becomes noticeably blurry with 1.00 Diopter of astigmatism or more.
No Astigmatism
1.00 Diopter Astigmatism
Astigmatism can be reduced during cataract surgery with a toric intraocular lens. You can easily perceive 0.50 Diopters of astigmastism and. If you have higher levels of astigmatism (2.00 diopters or more) the results of astigmatic management can be dramatic. See the astigmatism simulator for pictures of this.
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Whether or not to manage astigmatism may be the most important decision you make before cataract surgery. For some people, astigmatic management will not make a noticeable difference in their outcome. Others may expect a profound improvement in their vision.
If you have been told in the past that you have astigmatism, or have worn astigmatic correcting contact lenses, it is likely that you would benefit from astigmatic correction during cataract surgery. You can figure out how much astigmatism you can expect after cataract surgery if you do not choose to correct it. To do so, click here.
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Astigmatic management can be beneficial even if you intend on wearing glasses after cataract surgery. This is because fixing astigmatism inside the eye is optically the best way to correct astigmatism. It doesn't distort images like astigmatic correcting glasses or contact lenses do.