Mark Segal and Nick Devlin recently did a comparative review of the Pentax 645D versus the Phase One P40+. You can read the full review on the website Luminous Landscape. An interesting read, with the overall conclusion being that very little separates the two beasts with regards to image quality. This might be expected as you’re comparing two medium format cameras with 40 megapixels sensors, but the price difference between the cameras is significant. The Pentax seems the far better option as a field camera, while the ability of the Phase One to shoot tethered gives it the advantage in a studio.