While twin-lens reflex cameras have been more numerous in the medium format film category, many medium-format SLRs had been (and some still are) produced. Hasselblad of Sweden has one of the best-known camera systems utilizing 120 and 220 film to produce 6 cm × 6 cm (2¼" × 2¼") negatives. They also produce other film backs which produce a 6 cm × 4.5 cm image; a back which uses 70mm roll film, a Polaroid Back for instant 'proofs' and even a 35mm film back.
Pentax produces two medium-format SLR systems, the Pentax 645, which produces a 6 cm × 4.5 cm image; and the Pentax 67 series, which system evolved from the late 1960s introduced Pentax 6 × 7 camera. These Pentax 6 × 7 series cameras resembled huge 35mm SLR camera in look and function. In 2010 Pentax introduced a digital version of the 645, the 645D, with a Kodak-built 44X33 sensor.
Bronica (which has discontinued camera production), Fuji, Kyocera (which has also ceased production of their Contax cameras), Mamiya, Rollei, Pentacon (former East Germany), and Kiev (former Soviet Union) have also produced Medium Format SLR systems for a considerable period of time. Mamiya produces what is termed a medium format digital SLR. Other medium-format SLRs, such as those from Hasselblad, accept digital backs in place of film rolls or cartridges, effectively converting their film designs to digital format use.
In the case of Polaroid Corporation with its instant film line, the introduction of the Polaroid SX-70 was one of the few SLRs produced that was a rare case of a folding SLR.
Read more about this topic: History Of The Single-lens Reflex Camera