What causes astigmatism?

Irregular Cornea: Astigmatism usually is caused by an irregularly shaped cornea. Instead of the cornea having a symmetrically round shape (like a baseball), it is shaped more like a football, with one meridian being significantly more curved than the meridian perpendicular to it. Astigmatism usually causes vision to be blurred or distorted to some degree at all distances.

Eye growth: Mosty astigmatism is due to the way the eye develops as it grows and is due to differential growth in one axis causing an ovalness of the need for glasses. Some astigmatism can also be from eye trauma.

Refractive Error: Here is a great article by mayo clinic: http://www.Mayoclinic.Com/health/astigmatism/ds00230/dsection=causes. Astigmatism has to do with distortions of the curves in the cornea or the lens of the eye.

Cornea, lens: In astigmatism, light that enters the eye is refracted more in one direction than the other, allowing only part of the object to be in focus at one time. 95% of the time, this is caused by differences in curvature in different axes of the cornea on the front of the eye. The cornea does 2/3 of the focusing of light in the eye. The other 1/3 is done by the lens, which causes 5% of astigmatism.