Astigmatism is one of three classes of refractive error (the bending of light as it enters the eye). The eye is slightly "football shaped", and the light passes irregularly out of focus into the eye falling in front of, or behind, the retina. Images are out of focus at any distance.

The other two classes of refractive error are hyperopia (farsightedness), and myopia (nearsightedness). The eye is more "basketball shaped" and light falls long (behind) or short (in front) of the retina. These two classes are corrected with simple spherical lenses. If either is combined with astigmatism, a compound (toric) astigmatic lens is required.