In myopia and hyperopia the image of a distant object is brought to a single point focus in front of or behind the retina respectively. In the case of astigmatism however the cornea is more steeply curved in one direction (principal meridian) than the other which is, therefore relatively flat.
The classic analogy is the comparison of a soccer ball with a rugby ball. The soccer ball is a perfect sphere and looks identical regardless of the viewing angle. The rugby ball however has a long axis and a short axis and therefore appears oval or elliptical when viewed ‘from the side’ but circular when viewed ‘end on’. If the human cornea has an elliptical or oval shape the resulting optical defect or refractive error is astigmatism. Instead of a point focus, 2 line foci are produced and the spectacle lens required to correct this is termed an astigmatic, cylindrical, toroidal or toric lens.
Given the above explanation it follows that astigmatism must have not only a magnitude or amount (in dioptres) but must also have a defining direction, angle or axis (in degrees). Very often astigmatism will co-exist with either myopia or hyperopia.