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 (2" × 2") 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.