Fig. 12. Inherent lens performance errors for different base curves at an average fitting distance for a CR-39 plastic lens of power + 4.00 -2.00. Both the spherical and cylindrical errors were determined at two points 28 degrees from the optic axis. Point A was in the cylinder axis meridian, and point B was in the cylinder power meridian. Blur was defined as the absolute value of the largest of the four errors, with negative spherical errors weighted by 0.5 against radial astigmatism and plus power errors. (Modified from Davis JK: Prescribing for visibility. Probl Optom 2:131, 1990.)