Defects Of Vission (Astigmatism)

Astigmatism: This defect is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred. Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.

Credit: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/eyedef.html and          http://www.chm.bris.ac.uk/webprojects2002/upton/defects_of_the_eye.htm