Astigmatism occurs when the cornea is "out of round," such that a viewed image is not focused on a single plane. Astigmatism is very common, and the majority of patients who wear glasses or contacts have some degree of astigmatism, which can occur with nearsightedness or farsightedness.

Astigmatism can be corrected with spectacles as well as with contact lenses. Smaller amounts of astigmatism can be corrected with spherical soft contact lenses, and larger degrees of astigmatism can be corrected with rigid gas permeable contact lenses or special "toric" soft contact lenses.

Astigmatism can also be easily corrected with LASIK, corneal incisions or toric lenses in patients undergoing cataract extraction surgery.

Irregular astigmatism is a relatively unusual type of astigmatism typically associated with corneal diseases or occurring after corneal surgery. The correction of irregular astigmatism usually requires contact lenses or corneal laser surgery.