At times the eye is not able to focus the light coming from the horizontal and vertical planes. As a result, the horizontal and vertical views of an object will not be the same. Such a defect of the eye is called astigmatism.
Astigmatism occurs when the eye-lens or the cornea or both are not perfectly spherical in shape. In this case the image of a distant point object appears as a line.
Astigmatism is rectified using cylindrical lenses.
