	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.