Some essential optical features of the Zeiss fundus camera.

The imaging system of the Zeiss fundus camera comprises a front lens and a mirror with a central opening in the anterior focus of a camera objective. Focusing is achieved by changing the distance between the film and the camera objective. The principal point of the examined eye is imaged in the central opening of the mirror when the camera is correctly positioned. By applying reasoning current in elementary geometrical optics it was found 1) that both the absolute and the relative magnification depend on the "reduzierte Axenlängenkonvergenz" only, 2) that it is possible to correct the influence of refraction on magnification and 3) that the relation between the camera extension needed to provide sharp pictures and the principal point refraction of the examined eye is linear. The validity and practical importance of those findings as well as transcleral illumination and theoretical resolving power are briefly discussed.