Correction of optic disc measurements on fundus photographs

If defined as diameter of image/diameter of object, the magnification M of the eye-camera system is kB, where k is a camera constant and B represents the vergence of the internal axis of the eye. The magnification may be assessed (1) from the length l of the eye, using the formula 1.336 k/(l-0.0016); (2) from the glass refraction G, using the formula kD/(1-G/D), in which D is the "normal" refractive power of the eye; or (3) from the principal point refraction A and the deviation delta D1 from the "normal" refractive power of the cornea, using the formula kD.(1 + delta B/D), where delta B = A + 0.84 delta D1. If k is 0.042 m, as in the Zeiss camera, and D is about 60 dpt, the formulas may be written as (1) M = 0.056/(l-0.0016); (2) M = 2.5/(1-0.017 G); and (3) M = 2.5.(1 + 0.017 delta B).