An SLR mirror sits in the light path, in front of the film plane; to take a shot it must move out of the way, which blacks out the viewfinder. In many early SLRs, the mirror had to be returned — and the viewfinder restored — manually. Some models provided a fast returning mirror, which returns when the release button is raised, independently of the shutter cycle. Such systems were used on the Reflex-Korelle or Praktiflex, and on many older large-format SLRs. They relied on gravity or were driven by a spring, and required one to keep the button pressed until the exposure is completed.
A true instant-return mirror is controlled by the shutter mechanism, and automatically returns after the shutter is fully closed, whatever the position of the release button. This feature was first introduced on the Hungarian Gamma Duflex, whose first prototypes were made in 1946 and series production started in 1947. It seems that a Japanese 6×6cm SLR prototype called Orchid was also tried that feature in 1946. However the first camera produced in quantity with an instant-return mirror was the Asahiflex II, released in 1954. The introduction of reliable instant-return mirror mechanisms and the subsequent elimination of the "mirror blackout" was an important step in the acceptance of SLRs by a wide public.