That means the standard 35mm frame has "half" the diagonal of the 67 (though the ratio is different) but a quarter the area. This affects the focal length of lenses so that to get an equivalent field of view on a 35mm camera you need half the focal length. There is a similar effect on the depth of field of a particular aperture, so a 90mm f1:3.5 on the RB67 is equivalent to using a 45mm f1:1.8 on 35mm full frame. In 1975 Mamiya started to offer the M645, a camera re-using the 1930s 6x4.5 cm frame, allowing 15 shots on a standard 120 roll film becoming the first SLR MF camera to offer exclusively that format size also known as the 645 format. The RB67 was followed by the more advanced RZ67 6x7cm frame format camera in 1982. These cameras established Mamiya as a major medium-format professional camera manufacturer, together with Hasselblad, Rollei, Bronica and Pentax. In 1989, Mamiya introduced the Mamiya 6 (6x6cm) rangefinder camera. In 1995, this was followed by the Mamiya 7 (6x7cm). The Mamiya 6 was discontinued ca. 1999; the Mamiya 7 was produced for 18 years, with production ending ca. 2013. The Mamiya 6 is more portable due to a collapsible lens mount, while the Mamiya 7 offers a wider variety of lens options. Both the Mamiya 6 and 7 are compact and quiet cameras which are reputed for the extremely high optical quality of their lenses. In 1999, Mamiya presented the Mamiya 645AF, a 6X4.5 frame SLR camera with interchangeable lenses and film backs, auto focus and an integrated prism visor that would be the base platform for the Mamiya 645AFD film and digital back cameras. Digital products Mamiya introduced the Mamiya ZD, which was a compact medium-format camera, in 2004. Rather than taking the form of a digital back solution, it was all built into one unit, much like a 35mm camera. This camera utilized the Mamiya 645AF lenses and had a resolution of 22mp. The solution had technical difficulties and became delayed. At the same time, Mamiya also announced a ZD back which had the same specification but was intended to be used with the Mamiya 645AFDII / AFDIII. The ZD back was even more delayed and, once it was introduced, it was already outdated. In 2009, the Mamiya M Series digital backs were released (M18, M22 and M31) all featuring