Prismatic effects of spherical ophthalmic lenses

Exact prismatic effects were calculated for stock spectacle lenses (-6 D to +10 D) and trial refracting set lenses (-10 D to +10 D) using a range of visual field eccentricities on the 0.30 m Goldmann perimeter, and on the 1- and 2-m tangent screens. Slightly larger prismatic effects were found for the tangent screens than the perimeter, but the difference in results for the two tangent screen distances was negligible. For low power ophthalmic lenses (-2 D +2 D), the prismatic effects were smaller than the variation in apparent target eccentricity caused by variation in patient fixation distance. Paraxial estimates of prismatic effects using both thick and thin lens theory were calculated, but found to become more inaccurate as target eccentricity increased. Data are presented which can be used by clinicians to predict the alteration in the size of kinetic perimetric isopters when ophthalmic lenses are used to correct patient defocus resulting from ametropia or presbyopia.