If the positions of the principal points of the crystalline lens are conjectured, its equivalent power and that of the eye can be calculated as described from ocular dioptrics. The only data required are the subject's refractive error and the results of keratometry and A-scan ultrasonography. The method assumes the profile of the lens to conform to an arbitrary norm. If the unknown lens profile of a given eye differs from this norm, errors will arise. These are shown to be relatively small, and even in extreme cases unlikely to exceed +/- 1.0 D for the lens and +/- 0.5 D for the eye.