camera, a design which appeared much like the Minolta SR-7. Olympus Pen F The Olympus Pen F series was introduced and produced by Olympus of Japan between 1963 and 1966. The System consisted of the original Olympus Pen F, later the behind-the-lens metering Pen FT, 1966–1972; and the non-metered version of the FT, known as the Olympus Pen FV, which was manufactured from 1967 to 1970. The design considerations used were unusual. The camera produced a half-frame 35 mm negative; it used a Porro prism as a design-replacement for the conventional pentaprism thus producing the 'flat top' appearance; and the view through the viewfinder was of 'portrait' orientation' (unlike standard 35mm SLRs which had 'landscape' orientation). These half-frame cameras were also exceptional in that all used a rotary shutter, rather than the traditional horizontally travelling focal-plane shutter commonly used in other SLR camera designs. The camera was produced with various interchangeable lenses. The smaller image format made the Pen F system one of the smallest SLR camera systems ever made. Only the Pentax Auto 110 was smaller, but the Pentax system was of much more limited range in terms of lenses and accessories. Introduction of light metering Professional Photographers of the 1940s and 1950s time-period preferred to use hand-held meters such as the Weston or GE selenium cell light meters, and others which were common during these periods. These hand-held meters did not require any batteries and provided good analog readouts of shutter speeds, apertures, ASA (now referred to as 'ISO') and EV (exposure value). Selenium cells, however, could easily be judged for their light sensitivity by simply looking at the size of the cell's metering surface. A small surface meant it lacked low-light sensitivity. These would prove to be useless for in-camera light metering. Built-in light metering with SLRs started with clip-on selenium cell meters. One such meter was made for the Nikon F which coupled to the shutter speed dial and the aperture ring. While the selenium cell area was big, the add-on made the camera look clumsy and unattractive. In order for built-in light metering to be successful in SLR cameras, the use of Cadmium Sulfide Cells (CdS) was imperative. Some early SLRs featured a built-in CdS meter, usually on the front left side of the top plate, as in the Minolta SR-7. Other manufacturers, such as Miranda and Nikon introduced a CdS prism which fitted to