From this press release
Mamiya Digital Imaging is proud to announce our new M series digital backs, consisting of three new models: the M18, M22 and M31, the latest additions to our digital line-up.
The M series digital backs have all been designed for compatibility with Mamiya's 645AFDIII. Through use of the specifically designed digital back adapter HX701, the backs can also be attached to our famed RZ Pro IID. The M18 has a resolution of 18 megapixels and a CCD size of 44.2 x 33.1mm. The M22 has a resolution of 22.1 megapixels and a CCD size of 48.9 x 36.7mm while the M31 has 31.6 megapixels and a 44.2 x 33.1mm CCD. These large sized CCDs make the digital backs ideal for medium format photography. Each digital back can be purchased separately or in a system kit with the Mamiya 645AFDIII camera and Sekor 80mm F2.8 D lens. The M series backs are the quintessential tool for commercial photographers and studios, covering portraiture and on location work.
There is more information available here
- an interesting mix of sensor sizes and resolutions.