Retrofocus Wide-Angle Lens

Reflex cameras put a mirror between the lens and the film, and a conventional wide-angle lens needs to sit closer to the film than the mirror allows. Pierre Angénieux in Paris reversed the telephoto idea: a diverging group in front, a converging group well behind it, so the lens's focus lies further back than its focal length. **US 2,649,022**, "Wide-angle photographic objective lens assembly", Angénieux, filed 29. July 1950, granted 18. August 1953, describes an objective whose "distance along the axis between the back surface of the last lens and the focal plane … is relatively very large with respect to the focal length". (His US 2,649,021 of the same day is a different lens - a large-aperture projection objective.) **You build one on the bench**, measure its back focus against its focal length, and fit a mirror in the room it leaves.