A condition in which the front surface of the cornea of the eye is not truly spherical. Although the eye is perfectly healthy, the corneal surface has discrepancies in the curvature, so that magnification in one direction is greater than in others.
A minor degree of astigmatism is normal and glasses are not necessary to correct it. More severe astigmatism causes blurring of lines set at a particular angle. A person with astigmatism might see horizontal lines clearly but vertical lines blurred, or the blurring may occur in an oblique meridian.
Ordinary "spherical" eyeglass lenses cannot correct
astigmatism. Lenses are needed that specifically correct for astigmatism.
Hard or rigid contact lenses bridge over the anomalous corneal curve and
present a perfect spherical surface for focusing; they give excellent vision in
astigmatism. Ordinary soft lenses tend to mold to the astigmatic curve,
resulting in limited correction for a person with astigmatism. Special "toric"
soft lenses are available to treat astigmatism effectively.