Olympus Pen F
The Olympus Pen F, Pen FT and Pen FV are very similar half-frame 35 mm single-lens reflex (SLR) cameras with interchangeable lenses produced by Olympus of Japan between 1963-1966 (Pen F), 1966-1972 (Pen FT) and 1967-1970 (Pen FV). The original Pen F has a double-stroke film advance and a distinctive logo rendered in a gothic font. The later Pen FT added a single-stroke film advance, and an uncoupled, integrated light meter, which uses a system of exposure numbers rather than f-stops. The exposure numbers were added to the aperture  rings of later Pen F lenses; the rings could be pulled out and rotated to show conventional f-stops instead. A side-effect of the FT's light meter was a dimmer viewfinder. The Pen FV was essentially a Pen FT with the light meter deleted and the F's brighter viewfinder reinstated. "Half frame" means that the camera uses an 18×24 mm vertical (portrait) format, producing twice the pictures on a roll of 135 film as the regular 36×24 mm format. The smaller image format also allows for a smaller camera and lenses, making the Pen F system one  of the smallest SLR systems ever made; the Pentax Auto 110 was smaller, but with a much more limited range of lenses and accessories, and smaller 110 film. These cameras are somewhat exceptional since they used a rotary focal-plane shutter, rather than the two-curtain focal-plane shutter commonly used in other SLRs at that time. Since this one-piece shutter opens fully before it starts to close, it can synchronize to electronic flash at all shutter speeds. Pen-F series cameras are occasionally modified to mount standard motion picture camera lenses for use as film test cameras with  35mm motion picture films. The Pen-F frame size is close to the 35mm motion picture Super 35 frame.