Just to add info for the next person googling this topic…
I have a Phase One DF paired with a Phase One P65+, and now the S2. The handling between these two cameras is very different. The S2 is like a large dSLR and its viewfinder easily surpasses the Phase One DF in terms of brightness, perhaps size and clarity. To put this into perspective, one of my favorite lenses is the Mamiya 645M 200mm F2.8 APO. This is a totally manual lens and requires stop-down metering. On the Phase One DF my manual focus keeper-rate is around 10-20% at wide apertures. On my first outing with the S2 (using the Leica S / Mamiya adapter), after 174 pictures, 155 images where in focus to an acceptable degree. That’s 89%! Half of those pictures were with the Mamiya 200mm F2.8 APO.
The Phase One DF system is best suited on tripod (in my opinion) and using MLU. My handheld shooting results are not good and the lack of results led to the S2 purchase. With the Leica S2, I as popping off handheld shots easily. Zooming in on the rear LCD to check focus & sharpness is very doable; the LCD is much better than the rear LCD on the Phase One P+ series. The S2 auto-focus is slow, but it’s more accurate than the Phase One DF. A big difference in the shooting experience is that the Phase One P65+ base ISO is 50 and the S2 is ISO 160. So, there is a big, big difference in shutter speeds. I could boost the P65+ ISO to 100 or 200, but all of its 60 megapixel goodness is at ISO 50.
So, while the S2 wins in the handling department, the P65+ outputs better files. The files (at ISO 50) are cleaner, the color graduations are much better and hold much better in post processing, and the dynamic range is slightly better. Lightroom 4 blurs that dynamic range comparison a little bit, but I still think the ISO 50 shots from the P65+ have more headroom in the highlights. And Phase One’s Sensor+ performance is quite impressive. Shooting a P65+ back at ISO 800 or 1600 sort of defeats the purposes of all that quality, but the ISO 800 performance is — good! ISO 1600 is usable and looks fine in print.
My gut feeling is – if ergonomics are very important and you want to use the camera like a dSLR, get the S2. On the other hand, if file quality is “the” most important aspect and you’re willing to battle with the camera, then get the Phase back. I really like Phase One backs and find their files to be simply amazing, but their system is handicapped by the Phase One DF. I’ve used other Phase backs with the Mamiya 645AFD II, the difference between the 645AFD II and the DF in terms of the viewfinder and autofocus is slim and none.