Deciphering corneal astigmatism - calculation pitfalls and how to avoid them.

Options for correcting astigmatism include spectacles or contact lenses, corneal refractive laser surgery or relaxing incisions, toric lenses in the capsular bag, or implantation of supplementary toric lenses in the phakic or pseudophakic eye. This correspondence addresses some of the potential pitfalls which could cause calculation errors when working with astigmatic surfaces and spherocylindrical vergences and gives some advice on avoiding these. To provide some insight, each potential pitfall is illustrated with a clinical example.