Lack of symmetry in the curvature of the cornea or, rarely, the lens of the eye .
The unequal curvatures spread light rays, preventing them from being sharply focused at a point on the retina , causing blurring of part of the image. The effect of astigmatism can also be produced by misalignment of the lens. Astigmatic vision is corrected by means of lenses (see contact lens , eyeglasses ) that refract the light rays to the proper degree in the opposite direction of that produced by the defects in curvature.