There are two kinds of astigmatism: 1. lenticular and 2. corneal.

Astigmatism occurs when the surface of the cornea or of the lens is not perfectly smooth. If, for example, there is a small flat spot on your cornea the image in a certain direction on your retina will not be in perfect focus.

If the figure on the right was constructed with all the radial lines of equal sharpness and contrast a a person without astigmatism would see all these radial lines as perfectly sharp and with the same contrast. The diagram on the right has been fudged to   illustrate how it might appear to a person with astigmatism. 

This condition can be corrected with a corneal contact lens or with a cylinder lens in a pair of spectacles.

Visual Acuity
Table of Contents
Subject Index
Table of Contents [When not using frames]