The first 35mm SLR was the German Ihagee Kine-Exakta in 1936, which was fundamentally a scaled-down Vest-Pocket Exakta. This camera used a waist-level finder. Further Exakta models were produced before and during the Second World War, making the Exakta the first 35mm SLR system. The Ihagee factory in Dresden was destroyed by bombing in 1945.
Meanwhile, Zeiss began work on a 35mm SLR in 1936 or 1937[1]. This used an eye-level pentaprism which allowed viewing of an image oriented correctly left to right and while the camera was held up to the face. Waist-level finders show a reversed image while the operator of the camera has their head bowed downwards. To brighten the image, Zeiss incorporated a fresnel lens in between the ground-glass screen and pentaprism, forming the conventional SLR design still used today. However, the war intervened, and the Zeiss SLR did not emerge as a production camera until Zeiss in newly-created East Germany introduced the Contax S in 1949. This was the first "fixed" eye-level pentaprism SLR.
http://en.wikipedia.org/wiki/History_of_the_single-lens_reflex_camera
