I am most concerned about high resolution work on tripod. I assume that tripod is pretty necessary to make full use of the resolution, but i may be wrong.
Erik, I'm going to make my response in the context of big, focal plane shutter, cameras in general, rather than the Sony a7R, which is a bit of a red herring on this thread, since it has a set of shutter characteristics that don't apply to any medium format camera. WRT the 645Z, Lloyd's charts indicate to me that the camera;s shutter shock is quite a bit lower than the a7R's, although the star target that Lloyd uses makes getting any quantitative data off the image impossible -- at least for me. As an aside the crosses on the ISO 12233 chart are much better for this.
Enough preamble. On to your point. I'm assuming that anyone buying a 645Z wants to use all 50 MP; otherwise, why not get a D8x0, have a big choice of lenses, and save a bundle? One non-vibrational impediment to using all those pixels is focusing. In doing handheld work with the ISO 12233 target and a slanted edge target for SFR analysis, I was totally flummoxed when working with manual focus lenses. I gave it up and had to go with AF lenses, which are thin on the ground for the alpha 7 line. The reason was that with manual focus slight movements of the camera between focusing and shutter release resulted in focus errors that totally swamped out the resolving power of the lens and the camera. This was a particular problem with the a7R, since focusing had to be done at taking aperture when handholding, but would be a problem with a DSLR, which would have the advantage of the ability to focus wide open, but the disadvantage of a much less precise way to judge focus than live view.
So, leaving vibration out of it, I think you need a tripod to consistently use all that resolution. Electronic flash and AF lenses might ameliorate this. BTW, a lot of studio electronic flash systems have fairly long (>1ms for t.1) flash durations at some or all power settings.
Let's consider vibration. If you're shooting the 645Z handheld, you don't want to hold it at arms length for any length of time. That means you can't use live view. You want to be able to accurately frame the shot. That means you can't use mirror lockup. So you're going to have to suffer mirror slap. That alone is going to make it hard to use all those 50 MP. Hassy has a programmable delay between mirror up and first curtain release that apparently helps. I don't know if Pentax does.
As an aside, one thing that helped with electronic flash pictures with the a7R should also help on the 645Z: use trailing curtain synch and a longish (1/25 was a good number for the a7R) shutter speed.
All that is a long-winded way of saying that I agree with you.
Jim
