Fresnel vs. conventional prisms: their effects on the apparent fronto-parallel plane horopter

Among the inherent optical aberrations found in conventional crown glass or resin prisms is the property known as nonuniform relative magnification. Prisms, by definition, displace an image by a given amount. However, the final size of the image is nonuniform, being relatively larger toward the apex than toward the base of the prism. At present, reduction of the amount of nonuniform magnification is achieved by fabricating the prismatic correction on relatively steeper base curves (i.e., +9.00 D). This research deals with an attempt to reduce nonuniform magnification using Fresnel prisms. In this way, image displacement is created only by small prism apices, each of equal power and each producing identical image displacement resulting in a smaller degree of nonuniformity of image size. In this experiment, equal amounts of prism were fabricated on flat and steep base curves, using either conventional or Fresnel prism. The precise magnitude of the nonuniform relative magnification for each set of lenses was measured by a number of observers using the Apparent Fronto-Parallel Plane (AFPP) horopter apparatus. After statistical analysis using both an analysis of variance (ANOVA) and a matched pair t-test, there proved to be no significant reduction in the amount of nonuniform relative magnification when Fresnel prisms were used in place of conventional ground prisms. Furthermore, maximal reduction of this aberration appears to be effected only by fabricating such corrections on steep base curves. Relative magnification is more a function of the orientation of the base-apex meridian and the angle of incident light from an extended object than of effects of prism type or thickness.