A common form of visual impairment in which part of an image is blurred, due to an irregularity in the curvature of the front surface of the eye, the cornea. The curve of the cornea is shaped more like an American football or a rugby ball rather than a normal spherical basketball. Light rays entering the eye there are not uniformly focused on the retina. Rays entering through the more-curved surface are focused before the rays coming through the less-curved surface. The light is focused clearly along one plane but is blurred along the other. The result is blurred vision at all distances. Only part of what you are looking at is in clear focus at any one time.

Astigmatism may be so slight that it causes no problems. Almost everyone has some degree of astigmatism. Significant astigmatism can cause headaches and eye strain and seriously blur vision. Astigmatism may contribute to poor school performance but is often not detected during routine eye screening in schools. It is a refractive error, an error of focusing, that may coexist with other refractive errors like near- sightedness or far-sightedness.