I've been trying to answer for myself the question often asked in this thread: "Why do we need a 645D?"
(1) Print Size.
35mm digital is now at 18-24M pixels, which gives you excellent 12x18 prints, which can be pushed somewhat higher with upscaling. You only need 39M f you want to print high quality larger than 18x24 inches (0.50 x 0.75 meters). This is the realm of fashion, landscape, glossy magazines, calendars, and posters.
Professionals in fashion or publishing probably can already afford Hasselblad, PhaseOne and Leica. A 645D under $10,000 might be a sweet spot for the serious amateur landscape niche. I think wedding photographers are happy with the lenses, portability and ISO performance of Nikon or Canon.
(2) Is 35mm a dead end?
The other question might be whether 35mm sensors will continue to improve.
OGL's chart the other day indicates that 35mm at 24M pixels is starting to run into optical limits, specifically diffraction at smaller f-stops. In other words, 6 micron pixel size might be close to the physical limits given diffraction and DOF needs.
Just as in the age of film size matters, and landscape photographers, want to use f-11 or f-16 for depth of field, while resolving 40 or 50M pixels. Maybe Pentax would create a new market by offering a couple tilt & shift 645 lens?
(3) Physical and mechanical limitations.
With 6 micron pixel sensors, it looks like extraordinary lens quality, fine mechanical precision and pro-hardened design is necessary.
This interesting article by Joseph Holmes, a top-end landscape photographer (see his gallery), talks about just how much care you need.
quote:
Quote: "...the design properties of the cameras which can use the various medium-format and scanning back sensor options have, generally speaking, not fully caught up with the reality of the sensors. Most of the existing cameras were designed for either 645 film or 56 x 84 mm (6x9) film. The optical demands of 37 x 49 (or 36 x 48) mm sensors are vastly higher (four to six times higher in my estimation). So the precision required in the camera system is a huge issue and this includes not merely manufacturing precision, but also the things which allow the photographer to get a picture into focus in the first place, including the view through a viewfinder, the fineness of the focussing mechanism, the fineness of a tilt mechanism, where present, etc. The sheer size of the cameras also tends to be overly large, given the smaller size of the sensors."
In other words, 40-50M pixels on 645 sized sensors (not to mention 35mm at 18-24M pixels ) requires pro-quality bodies, lenses and focussing. I have some 645 Pentax lenses so I'd like to believe Pentax can compete with Leica and Zeiss lenses.