Pentaprism.
A glass prism with 5 sides (hence the name), found inside SLR cameras as part of the viewfinder optical system.
Pentaprisms were one of the key technical breakthroughs which permitted the creation of SLRs and their through-the-lens viewing capabilities. The 1949 Zeiss Ikon Contax S was the first SLR with a built-in pentaprism.
These solid glass prisms direct light up from the lens into the viewfinder, and account for the large, bulky and characteristically pyramidal shape of the top of SLR cameras. They not only permit the photographer to view the scene through the lens while focussing (useful for previewing depth of field, filter effects, etc.) but they also allow the viewfinder image to be shown both right-side up and the right way around. By contrast, twin-lens reflex cameras reverse the image in their viewfinders and view cameras both reverse them and turn them upside-down.
cf. Contax, prism, reflex mirror, roof mirror, single lens reflex (SLR), twin-lens reflex (TLR), view camera, viewfinder.
Entry last updated 2002-04-16. Term 892 of 1487.
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