ASTIGMATISM

lack of symmetry in the curvature of the cornea or, much less commonly, of the crystalline lens (the cornea is the transparent wall of the eye in front of the pupil and iris). The uneven and unequal curvatures on these various refractive surfaces diffuse light rays and interfere with a sharp focus at a point on the retina, the light-sensitive tissue lining the back and sides of the eyeball. The result is blurring of part of the image on the retina. The effect of astigmatism can also be produced by misalignment of the lens. Astigmatic vision is corrected by means of lenses that refract the light rays to the proper degree in precisely the opposite direction of that produced by the defects in the refractive surfaces of the cornea or lens of the eye.