was an aperture-priority-only SLR; the ZE-2 added manual exposure; the ZE-X added shutter priority and full program automated mode, and (with a dedicated flash and an EF-series lens) focus-priority flash exposure). In these models the aperture ring had no direct connection to the diaphragm, allowing the camera body to override the set aperture, and the lenses could communicate a considerable amount of information to the camera body via electrical contacts on the mount. The Mamiya ZM, introduced in 1982, was essentially an advanced version of the ZE-2, with some of the features of the ZE-X. It was the last Mamiya 135-film camera produced. It had an aperture-priority automatic time control, based on center-weighted TTL readings, an automatic shutter-speed range from 4 seconds to 1/1000, and a manual range from 2 seconds to 1/1000. Visual and audio signals indicated over- or under-exposure, pending battery failure, or excessive camera shake. Metering modes, shutter release, self-timer, manual time settings and the ergonomics of the camera body were also improved. In 1984 Osawa, one of Mamiya's major distributors, filed for the Japanese equivalent of bankruptcy and, soon after, Mamiya discontinued 135-film camera production to focus on the medium-format professional market. Medium format Mamiya made a series of square format twin lens reflex (TLR) throughout the middle of the twentieth century. These were developed into the C cameras (C2, C3 through to C330s) which have interchangeable lenses as well as bellows focus. In 1970, Mamiya introduced the RB67 6×7 cm professional single lens reflex (SLR). The RB67, a large, heavy, medium-format camera with built-in closeup bellows was innovative and successful. Previous medium-format professional SLR cameras used the square 6×6 cm format which did not require the camera to be rotated for photographs in portrait orientation, problematical with large and heavy cameras when tripod-mounted. Like the Linhof Technika the RB67 had a rotating back which enabled photographs to be taken in either landscape or portrait orientation without rotating the camera, at the expense of additional weight and bulk. The RB67 soon became widely used by professional studio photographers. The 6x7 frame had been introduced and patented by Linhof (56 x 72mm) and was described as being ideal, as the negatives required very little cropping to fit on standard 10" x 8" paper. Mamiya actually used 56 x 67mm. When comparing the RB67 to full frame 135 cameras there is a so-called "crop factor" of a half.