"A half-frame camera is a camera using a film format at half the usual exposure format. A common variety is the 18×24 mm format on regular 135 ...
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film. It is the normal exposure format on 35mm movie cameras. For still cameras using the 35mm film, the usual format is 24×36 mm, so still cameras taking 18×24 mm exposures are called half-frame cameras.
There was a vogue of half-frame cameras in the 1960s, mainly from Japan, originating with the Olympus Pen models. It allowed for a very compact camera, using commonly available film, unlike other subminiatures that used exotic films (16mm, 9.3mm, etc.).This vogue ended when cameras like the Rollei 35 or the Olympus XA showed that it was possible to make cameras as small as the half-frame ones, but taking 24×36 mm exposures." - (en.wikipedia.org 28.03.2020)
