Originally posted by Digitalis
44X33mm sensors are rather common in MF digital backs, Full frame 645 is somewhat of a niche area. Mostly due to the steep increase in cost - not only in the camera but also on lenses that have decent edge-to-edge image quality.
To say honest, digital MF cameras are not rigidly bound with 56*41.5 mm film size.
Crop-factor is just for convenience of lens angle's understanding. I would say that 44*33 mm is digital standart for MF cameras.
There are 2 sensors almost 56*41.5 mm (53.7*40.4 - 80 MP and 53.9*40.4 mm - 60 MP), but expensive and it's rather special.
44*33 mm CMOS sensor could be more advanced than the same size CCD. And it could be much better in terms of noise than 60 and 80 MP CCD sensors.