“The triggering mechanism that released a flood of new lens designs was the emergence of the 35-mm SLR as a practical, rapid, and flexible camera during the mid-1950s. Then the Contax S from East Germany introduced eyelevel camera operation by incorporating a pentaprism for the first time, and the Asahiflex II from Japan ushered in the instant-return reflex mirror. Once these two features were combined with the gradually evolved auto-diaphragm, the 35-mm SLR became the vehicle for most of the new and interesting lens design.”

Norman Goldberg, “The Miracle of Modern Lenses”, The Best of Popular Photography (1979)