The instant-return mirror
1954 (Asahiflex IIb)
A spring brought the SLR mirror straight back down after each exposure, so the viewfinder went black only for an instant instead of until the film was wound.
How it began
In early SLRs the mirror flipped up for the exposure and stayed up until the film was wound, leaving the photographer blind. The Hungarian Gamma Duflex of 1947 had tried a returning mirror; in 1954 Asahi Optical, later Pentax, put one into its Asahiflex IIb.
How it works
Releasing the shutter swings the mirror up out of the light path. As soon as the second shutter curtain closes, a spring pulls the mirror back down, restoring the view through the lens.
Why it mattered
The blackout shrank to an instant, so the photographer could keep watching the subject. With the pentaprism, it made the SLR as quick to use as a rangefinder and helped Japanese makers take over the market.
Limits and what replaced it
The moving mirror causes noise and vibration, and blocks the view during the shot. Mirrorless cameras removed it entirely.
Today
Every SLR made since the 1960s has one; mirrorless cameras have no mirror to return.