The cornea of an astigmatic eye is shaped like a torus, sort of like a rugby ball, while a spherical form like a tennis ball is normal. This deformation diverts beams of light headed towards the retina, which in turn distorts image interpretation in the brain. Astigmatism is considered to be weak between 0.25 and 1.50 dpt, medium between 1.50 and 3.00 dpt, and strong above 3.00 dpt.
If the toric shape of the cornea is the only cause of astigmatism, this is referred to as corneal astigmatism, If the cause is an irregularly shaped lens, this is called lenticular astigmatism. Total astigmatism is when both of these phenomena occur together.