a very smart design with a 24mm × 36mm frame size, but, according to some sources, it did not enter the market until 1937, although there is now some evidence emerging that "Sport" may have been in limited production before 1936. Therefore, it is claimed not to be the first mass produced 35mm SLR. Early innovations Early 35 mm SLR cameras had functionality that was similar to larger models, with a waist-level ground-glass viewfinder and a mirror that remained in the taking position and blacked out the viewfinder after an exposure, and then returned when the film was wound on. Innovations that transformed the SLR were the pentaprism eye-level viewfinder and the instant-return mirror—the mirror flipped briefly up during exposure, immediately returning to the viewfinding position. The half-silvered fixed pellicle mirror, without even the brief blackout of the instant-return mirror, was also innovative but did not become standard. Through-the-lens light metering was another important advance. As electronics advanced, new functionality, discussed below, became available. Exakta The first 35mm format SLR in large scale production was the Ihagee Kine Exakta, produced in 1936 in Germany, which was fundamentally a scaled-down Vest-Pocket Exakta. This camera used a waist-level finder. Various other models were produced, such as the Kine-Exakta, the Exakta II, the Exakta Varex (featuring an interchangeable pentaprism eye-level viewfinder and identified in the United States as the 'Exakta V'), the Exakta Varex VX (identified in the United States as the 'Exakta VX'), the Exakta VX IIa, the Exakta VX IIb, the Exakta VX500 and the Exakta VX1000. Ihagee also manufactured less expensive cameras under the 'Exa' camera label, such as the Exa, the Exa Ia, the Exa II, the Exa IIa, the Exa IIb (which was generally not considered part of the "official" Exa line), and the Exa 500. The Exacta sold well and triggered other camera manufacturers to develop 35mm SLRs. Sales were particularly strong in the medical and scientific fields. A large range of lenses and accessories were made by a variety of manufacturers, turning the camera into one of the first system cameras, although motor drives and bulk loading backs were never produced by Ihagee. Rectaflex Rectaflex was the name of an Italian camera maker from 1947 to 1958. It was also the name of their sole model. The Rectaflex was a 35 mm SLR camera with a focal plane shutter, interchangeable lenses, and a pentaprism eye-level finder.