Bob, did you calibrate your monitor for brightness ?
In the dark areas on the ISO400 shots they are seen very clearly on my monitor.
If you want to make sure, crank up your brightness about 20% and you will see what I mean.
It's not noise, noise is not bad, in the early day it was called grain (well ok it's different I know), I never have had a problem with noise, sometimes it really adds to a picture.
HOWEVER this purple wurms I'm calling them show up in skintones and in other colors, it's not noise but it's like a stamp on the pictures, for me that's a real problem, my customers want a clean image colorwise, they don't care about a little noise, that can be killed by many many plugins.
However these purple artifacts are not.
Ofcourse you don't have to shoot on ISO400 on a MF back, HOWEVER the problem I have is that when you pull shadows on an ISO100 file you can also see them, especially in skin tones which drove me nuts.
Also the strange thing was that on my first back I did not have a problem.
The main concern for me is and I'm torn between this.
Mamiya claims that it's ONE batch only with problem, now I see alot of ZD back files and on alot of them I can point the purple artifacts out.
Mamiya has always been very honest with me during my test time with the ZD back, however I'm now in doubt what is happening, it's not as bad as with my back what you show, but they are clear as day, and even on ISO400 or even ISO800 (pulled from ISO400) it should NOT happen.
Noise on ISO400 or ISO800 is quite normal, even pattern noise is normal (although irritating) but this is something else.
Maybe it's a killer test but try to shoot a dark subject on ISO400 and underexpose one stop, now get in ACR and get the exposure one stop up, it will show MUCH more clearly.
Of course people will say, you don't do that, well ok I agree but the problem is that the purple is also seen on anything above ISO125 on my last back and it got increasingly worse when I got to ISO400 where it was clear as day.
There was some talk about the 5D a while ago where people would underexpose an ISO3200 shot by 2 stops pulled it up and complained about PATTERN noise/banding in shadows.
Again that is normal and can be fixed.
This I don't know what it is must be a fault in their filter or software and I think that if more people report it they can and will fix it.
It's a killer product for a killer price, but this is something they should fix or indeed limit the back to ISO100.