Astigmatism

The eye differs from the camera, optically, in that it is not necessarily radially symmetrical [see refractive error].  If one were to examine two perpendicular cross sections of the eye (e.g. horizontal and vertical) the length of each would be identical, as would the optical power if the eye had no astigmatism.  For an eye without astigmatism the front surface of the cornea is spherical.  However, for many people the cornea’s front surface is actually shaped like a rugby ball, or an American football (see the picture below).  As a consequence of the different curvatures in the two perpendicular meridians (i.e. the long and short axes of the rugby ball) the optical power in each meridian differs, and the person is said to have astigmatism (or is said to have an astigmatic eye).  The normal eye brings light to a point focus (‘stigma’ means ‘point’) whereas the astigmatic eye does not.
Astigmatism can be found either by itself, or in conjunction with myopia or hypermetropia. Most people have a small amount of astigmatism, but do not suffer severe consequences.  Some people, however, will complain of headaches and ‘eyestrain’ which is relieved by the optical correction of the astigmatism.

You can test whether you have any astigmatism by looking at the diagram of the fan chart.  If all lines look equally black, with each eye separately, then you are unlikely to have any astigmatism.  However, if any lines look blacker than the rest - and the blackest lines change as you rotate your head around an anterior-posterior axis - then you probably do have some astigmatism.
 
 

Perceptually, most astigmatic people do not suffer any distortion of the visual world, and are generally unaware of any visual consequences of the astigmatism.  However, correction of the astigmatism will often lead to temporary perceptual distortion with straight lines appearing to be slanted.  Visual adaptation to the distortion produced by the wearing of a new optical prescription is generally fairly rapid, and after a few days the world will look ‘normal’ again.
[See also misuse of terms].