Figure 4-6 Determining the power of a contact lens using the FAP-SAM rules. (Illustrations developed b y Thomas F.

Mauger, MD.)

Correcting Astigmatism

Because rigid (and toric soft) contact lenses neutralize astigmatism at the corneal surface, the meridional aniseikonia created by the 2 different powers incorporated within each spectacle lens is avoided. For this reason, contact lens–wearing patients with significant corneal astigmatism often experience an annoying change in spatial orientation when they switch to spectacles. However, refractive astigmatism is the sum of corneal and lenticular astigmatism. Lenticular astigmatism, if present, is not corrected by spherical contact lenses. Because lenticular astigmatism usually has an against-the-rule orientation (vertical axis minus cylinder), it persists as residual astigmatism when the corneal astigmatism component is neutralized by rigid contact lenses. This finding is more common among older patients and often explains why their hard contact lenses fail to provide the anticipated