the universal standard frame size until the early 1950s. Note that 135 film cameras using non-standard frame sizes, such as 24×18 mm or 24×24 mm, continued to be made into the early 1990s. Panoramic 135 film cameras using extra-wide aspect ratio frame sizes (up to 24×160 mm for the 360° revolving slit Globuscope [USA] of 1981) were still available in 2006. The Sport (camera) is the series production model of a prototype camera called Gelveta. The Gelveta was designed and built by A. O. Gelgar between 1934 and 1935. It is the earliest known 35mm SLR camera ever to be built, but fewer than 200 examples were made. It was manufactured by the Soviet camera factory Gosudarstvennyi Optiko-Mekhanicheskii Zavod, The State Optical-Mechanical Factory in Leningrad. GOMZ for short. The camera name is engraved in Cyrillic on the finder housing above the lens: „Спорт“. The manufacturer's prism logo in gold on black with the factory initials ГОМЗ (GOMZ) is shown behind a circular magnifying window on the top left camera front. An estimated number of 16,000 cameras were made 1936 Ihagee Kine Exakta (Germany): first production 35 mm SLR, first system SLR, first interchangeable lens camera with bayonet lens mount. This was exhibited at the Leipzig Spring Fair in March and was in production by April 1936. Had left-handed shutter release and rapid film wind thumb lever, folding waist level finder and 12 to 1/1000 second focal-plane shutter. Well-integrated design with excellent interchangeable lenses and good accessory system. Fewer than 30,000 Kine Exaktas were made before World War 2 stopped production in 1940. Production of improved models re-started after the war and Exakta was among the best known 35 mm SLR brands through the 50s. 1936 E. Leitz PLOOT (Germany): first reflex housing for 35 mm rangefinder cameras. For use with a Leica IIIa RF and the Leitz 20 cm f/4.5 Telyt or 40 cm f/5 Telyt long focus lenses (all Germany). Long focus (and telephoto) lenses have very shallow depth of field and the short baseline rangefinders built into RF cameras cannot triangulate the subject distance accurately enough for acceptably sharp focusing. SLRs do not suffer from this problem, because they are focused by directly assessing the sharpness of the lens image – the lens serves as its own rangefinder.<ref>Goldberg, Camera Technology", pp. 12–26</ref> Reflex housings converted RFs into very awkward SLRs by inserting a reflex mirror and focusing screen