The peripheral prismatic effects of corrective spectacle lenses introduce an artifact when strabismic deviations are measured. Plus lenses decrease and minus lenses increase the measured deviation. This effect begins to become clinically significant with corrective lenses with powers of more than +/- 5 diopters. We developed a simplified model that predicts the magnitude of this effect. The model agreed with actual ray tracing analysis through commonly used spectacle lenses and also agreed closely with clinical data. The clinical use of this model is facilitated by remembering the artifacts produced by +20, +10, -10, and -20 diopter spectacle lenses and interpolating for intermediate values. Correcting for these artifacts should enhance the predictability of strabismus surgery in patients with significant ametropia.