<prompt> Astigmatism is due to -Irregularity of curvature of corneaIrregularity of curvature of lensForward displacement of the lensBackward displacement of lens  <response>: 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 