Both assertion and reason are true but reason is not the correct explanation of assertion. Astigmatism is a defect of vision in which the image of an object is distorted, usually in either the vertical or the horizontal axis, because not all the light rays come of a focus on the retina. Some parts o f the object may be in focus but light from other parts may be focused in front of or behind the retina. This is usually due to abnormal curvature o f the cornea and/or lens whose surface resembles part of sphere. The defect can be corrected by wearing cylindrical lenses, which produce exactly the opposite degree of distortion and thus cancel out the distortion caused by the eye itself.