Astigmatism

understanding_astigmatism_basics_astigmatismAstigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. The two types of astigmatism are regular and irregular. Irregular astigmatism is often caused by a corneal scar or scattering in the crystalline lens, and cannot be corrected by standard spectacle lenses, but can be corrected by contact lenses. Regular astigmatism arising from either the cornea or crystalline lens can be corrected by a toric lens. A toric surface resembles a section of the surface of an Rugby ball or a doughnut where there are two regular radii, one smaller than the other one. This optical shape gives rise to regular astigmatism in the eye.

And since people with astigmatism can suffer from myopia or hyperopia, they can also be used to correct either of those conditions at the same time as correcting astigmatism.