The pentaprism eye-level viewfinder
1949 (Contax S; Rectaflex shown 1947–48)
A five-sided prism above the focusing screen turned the reflex image the right way up and round, so an SLR could be used at eye level like a rangefinder.
How it began
Early 35mm SLRs such as the Kine Exakta were viewed from above, and the image was reversed left to right. In 1949 the Contax S, made by Zeiss Ikon in Dresden, put a pentaprism on top of the screen; the Italian Rectaflex had been shown with one shortly before.
How it works
The mirror throws the image up onto the focusing screen. A five-sided prism with a roof edge reflects it several times inside the glass, turning it right way round, and passes it out through the eyepiece at eye level.
Why it mattered
Photographers could hold an SLR to the eye, follow moving subjects naturally, and see exactly what the lens saw. Combined with the instant-return mirror, this made the 35mm SLR the dominant camera from the 1960s.
Limits and what replaced it
Prisms are heavy and give a dim view at small apertures. Electronic viewfinders, which show the sensor’s image directly, replaced them in mirrorless cameras.
Today
Optical SLRs still use pentaprisms; mirrorless cameras replace them with electronic viewfinders.