Lloyd Chambers has just commented that he wouldn't purchase a Pentax 645Z because of poor and inconsistent lens quality (and quotes his testing on the 645D, as well as with the 25 mm. on the 645Z).
experience with the lenses for the Pentax has been different - centre
resolution has been just fine, and corners have been good across the
board (35A, 75A, 120A, 150A, 200FA, 300FA) and adequate on the 25 (entirely expected given the nature of the lens).
two facts though - first Lloyd is comparing the Pentax with its
available lenses, to the Nikon D810 with the best Zeiss glass, all
primes, all expensive, all manual focus. That's fine, but doesn't
reflect the real world for most who feel that the whole point of
something like full frame digital slr's is to have auto focus and
perhaps IS and zooms. Second, I looked into Pentax 645 glass before
making purchases and knew pretty much what I was getting into - I
settled on primes (not the zooms that Lloyd felt were so so), and
avoided focal lengths that had proven less than stellar (45 mm. and
oddly, the newish 55), and with the exception of replacing the 200A with
the 200FA, have been happy with my choices.
Shutter shake was an issue for longer lenses, not the lens quality itself when Lloyd did his 645D testing, and I too am having some shake issues. It isn't clear yet if this is the shutter or as I hope, the two second mirror up self timer that ideally should be at least 4 seconds. Oddly, when on the weekend I tried switching to the 12 second self timer, I lost mirror lockup. Next weekend I'm going to try the separate mirror lock up control PLUS the 12 second self timer and see how that does. I'm fairly optimistic this will fix the problem with the 200 and 300 mm. lenses as the shake I have seen is in random directions, not that of the shutter movement, and also based on my experience with the Canon 5D3 and Nikon D800e where I know damn well that 2 seconds was not enough to dampen vibrations with the 70-200 lens despite a good tripod, good head and good lens collar.
The 25 mm.
Pentax DFA isn't ideal, but neither was the Ziess 15 nor the Nikon
14-24. I suspect I could get close to perfection with a non retrofocus
lenses for technical cameras and a separate medium format back - for a
minimum of $25,000 for a one lens outfit. That isn't going to happen. I have made several images with the 25 that I could not have made with longer lenses, not without stitching anyway and given I was focus bracketting, that would have been cumbersome at the least.