Analysis of clinical approximation in applying Prentice's rule to decentration of spherocylinder lenses.

A common approximation for obtaining the prism induced by decentration of spherocylinder lenses in non-principal meridia has been compared to the actual prism for numerous cylinder axis orientations. The base direction predicted by the approximation shows significant variations from the actual direction when the cylinder axis is more than 20 degrees from the horizontal or vertical direction. When there is only vertical decentration, the power error is directly proportional to the decentration and the cylinder power. Both the meridional error and the power error depend on the ratio of the sphere power to the cylinder power and on the decentration.