A new method for determining prismatic effects in cylindrical spectacle corrections

This study presents a new method of finding differential prismatic effects in an anisometropic spectacle correction containing cylindrical lenses. The calculations are based on dynamic spectacle magnifications rather than Prentice's rule. If an imaginary circular object is considered, cylindrical lenses will produce elliptical far point images, which can be superimposed on the spectacle plane for comparison. The difference between left and right ellipses, in any meridian, then represents the distance the eyes have to diverge in order to fuse the object of regard. This distance can be translated into prism diopters of differential prismatic effect. Although the conventional methods for finding this effect often result in very large errors, the new method can be performed with great accuracy. In part, this is because it uses the actual eccentricities of the two eyes rather than an assumed average eccentricity. Moreover, the method includes considerations of base curves and center thickness. Contrary to the classical methods, it can thus be applied to clinically realistic lenses.