interchangeable viewfinder, first interchangeable focusing screens, first viewfinder condenser lens SLR.Matanle, pp. 52–54 Original viewfinder selection was waist-level or pentaprism. For the next half-century, interchangeable viewfinder customization was the signal feature of fully professional level SLRs, although they have not made the transition to digital SLRs. 1950 Angénieux 35mm f/2.5 Retrofocus Type R 1 (France): first retrofocus wide angle lens for 35 mm SLRs (for Exaktas).Herbert Keppler, "SLR: Are the sacrifices we make to use an SLR worth it?" pp. 27–28, 30, 34. Popular Photography, Volume 64 Number 6; June 2000. Regular wide angle lenses (meaning short focal length lenses) need to be mounted close to the film. However, SLRs require that lenses be mounted far enough in front of the film to provide space for the movement of the mirror – the "mirror box." Therefore, the focal length of early 35 mm SLR lenses were no less than about 40 mm. This prompted the development of wide view lenses with more complex retrofocus optical designs. These use very large negative front elements to force back-focus distances long enough to ensure clearance.Kraszna-Krausz, pp. 1675–1676 Note, "retrofocus" was an Angénieux trademark before losing exclusive status. The original generic term is "inverted telephoto." A telephoto lens (multiple inventions, 1891) has a front positive group and rear negative group; retrofocus lenses have the negative group in front and positive group to the rear. The first inverted-telephoto imaging lens was the Taylor-Hobson 35mm f/2 (1931, UK) developed to provide back-focus clearance for the beamsplitter prism used by the full-color via three negative Technicolor motion picture process. Retrofocus wide angle prime lenses reached fields of view as wide as 118° with the Nikkor 13mm f/5.6 (Japan) lens for Nikon 35 mm SLRs in 1975, but they are extremely large compared to non-SLR short focal length lenses because of their gigantic negative elements.Steve Sint with Peter Moore, "Fantasy Glass: Longer than long, faster than fast, wider than wide, made of pure golden glass, optics of unbelievable promise and price are being produced by major lens makers. Are they all hype … or do they perform?" pp. 44–49. Modern Photography, Volume 48, Number 2; February 1984. 1952 Zenit (Soviet Union, Russia; Зенит): first Russian pentaprism eyelevel viewing 35 mm SLR. 1952 Asahiflex I (Japan): first Japanese 35 mm SLR. Had folding waist level finder and focal-plane shutter.Cecchi, pp. 32–33Jason Schneider, "Camera Collector: What was the first 35mm