zeiss had begun work on a 35mm slr camera in 1936 or 1937. this camera used an eye-level pentaprism, which allowed eye-level-viewing of an image oriented correctly from left to right. waist-level finders, however, showed a reversed image, which the photographer had to mentally adjust for, while composing the image by looking downward and viewing and focusing. to brighten the viewfinder image, zeiss incorporated a fresnel lens in-between the ground-glass screen and the pentaprism. this design principle became the conventional slr design used today.