Think of how a tennis ball looks you squeeze it. The curvature becomes greater in one dimension, and is less in the other dimension. When this difference in curvature occurs in the cornea, astigmatism is present. Objects are blurred and distorted.
A more technical explanation of astigmatism can be found here.
When you get your eyes tested for glasses, you are able to detect 1/4 (0.25) Diopter of astigmatism. Some people can detect even less. Small amounts of astigmatism, up to 0.50 Diopter, probably will not bother you during your normal activities. Vision becomes noticeably blurry with 1.00 Diopter of astigmatism or more.
1.00 Diopter Astigmatism
Astigmatism can be reduced during cataract surgery with a toric intraocular lens. There is about a one in three chance that you have enough predicted postoperative astigmatism (1.00 Diopter or more) to benefit reliably from astigmatic management with toric lenses. 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.
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.
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.