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.
