If you shelled out for the $5,990 Mamiya Phase One 645DF, you probably have the funds to support your photography habit. Mamiya Leaf wants you to expand your arsenal, offering three new digital backs to go with that medium-format camera. The Leaf Credo 40, 60 and 80 are named for the number of pixels their CCD sensors are packing, and all have a dynamic range of 12.5 f-stops. Each sports a 3.2-inch, 1.15-megapixel touchscreen that extends beyond the edges of
