The astigmatism is a refractive defect, caused by an alteration of the curvature of the cornea, which is no longer similar to that of a football, but is rather similar to that of a rugby ball and, therefore, is not the same on the different meridians. From this the incapacity of the eye derives to focus on the image of an object or a person.
Before reaching the retina, the luminous rays coming from the external world must cross the cornea and the lens and it is, therefore, necessary that these natural lenses possess a correct focusing power. In particular, the total astigmatism of the ocular diopter is the result of the various combinations of astigmatisms of the cornea and of the lens.
Astigmatism can have origin, in fact, both from the cornea and from the lens: Therefore, visual alterations will occur both if the cornea presents an irregular sphericity and if the surface of the lens is not perfectly spherical or its optic axis does not correspond to the visual axis.