Olympus Pen F
The Olympus Pen F system was a one of a kind, half frame SLR system introduced in 1963. The size of the frame is exactly half of that on normal 35mm cameras. The Olympus Pen F SLR featured a vertical mirror and interchangeable lenses which were very uncommon for a half-frame camera. The benefit of shooting a half-frame SLR was the ability to shoot twice the amount of frames per roll while still having the framing accuracy of a single lens reflex camera.
The system also benefited from being tiny compared to 35mm SLRs, as the mirror was half the size and the body thickness, therefore, resembled a rangefinder more than an SLR. The Olympus F mount featured 18 different lenses. Lenses are adaptable to mirrorless cameras with an APS-C sized sensor.