Astigmatism is a defect or imperfection in the eye curvature of the cornea or in the structure of the lens of the eye, characterized by a different eye refraction between two meridians, which prevents sharp focus objects. The cornea acts as a lens through which light passes that is focused on the retina. So that if the surface of the cornea is irregular in its curvature, astigmatism occurs because the light rays are not refracted properly. Low ( 0.25 diopters less than 1 ), medium (1 diopter less than 3 ) and high ( greater than or equal to 3 diopters ): According grades three types are classified.