What is Astigmatism

Astigmatism is a refractive error caused by a non-spherical surface of the cornea or lens. Normal (non-astigmatic) eyes have a spherical cornea, like a basketball. Eyes with astigmatism have non-spherical corneas and are torodial, like a football. This means that the surface is more curved in one direction than it is in the direction 90° away. An eye with uncorrected astigmatism cannot refract focal points on the retina; it instead refracts two foci, one horizontal and one vertical, on or near the retina.

Astigmatism can be corrected through glasses, contact lenses, or surgery.  The most common way to correct astigmatism is with spherocylinders used in glasses or contact lenses. The most common directions for astigmatic correction are at the vertical meridian (90°) and the horizontal meridian (180°).

One way to tell if you have astigmatism is to look at your glasses or contacts prescription. If your prescription includes only one number per eye, a spherical number, than your lenses are not correcting for astigmatism. However, if your prescription has three numbers per eye, a spherical number, a cylindrical number, and an axis number, than your lenses do correct for astigmatism. You can tell how much astigmatism based on how far the cylindrical number is from zero. The farther it is, the more astigmatism it corrects for.