Astigmatism is due to -
Astigmatism is a type of refractive error wherein the refraction varies in the different meridia. Consequently, the rays of light entering in the eye cannot converge to a point focus but form focal lines. Broadly, there are two types of astigmatism: regular and irregular Corneal astigmatism is the result of abnormalities of curvature of cornea. It constitutes the most common cause of astigmatism Reference : A K KHURANA OPHTALMOLOGY,Edition 4,Page-34
Irregularity of curvature of cornea
Irregularity of curvature of lens
Forward displacement of the lens
Backward displacement of lens
Optics and refraction