This is from the DxO Mark scores, on the IQ180. They do rate the Phase iso's of 35, 50 and 100 as all at iso 29.
The problem as I see it is that photographers tend to use the same logic for iso increases on CCD and CMOS. The issue on whether the iso on the Phase One CCD is really an increase? or is it just a metadata setting for C1 to use later on. Unlike a CMOS iso increase which as I understand it, is an actual increase in gain on the chip.
There have been several comparisons of IQ180 iso 35 and iso 200 shots. I tried to find the one done a few years ago, but was not able. But the net, was that the image looked the same between a under exposed iso 35 by 2.5 stops and a correctly exposed. The two images when viewed side by side look pretty much the same noise levels etc. (Note I may have this backwards, i.e. the iso 35 was even exposed but the iso 200 was under exposed)
I have put this question forward several times on various forums but it's never really been answered very clearly. From reading various posts by users (as I have never found anything written officially by Phase One) is that you really don't change the gain on a CCD by pushing the iso but I am not qualified enough to know the answer.
I my use, (P45+, IQ160, IQ260, and IQ180), I feel that you can "push" the CCD maybe one stop, So on my IQ260, that would be either iso 50 to iso 100 or iso 100 to iso 200 depending on where you feel the real base iso rests. In normal exposure ranges say 1" to 1/250th most times I will not see much difference between iso 50 or iso 100 on my 260. What makes the difference to me is say the difference in shutter speeds, say 1/60 vs 1/125 at F11. Just that one shutter speed difference can make a big difference in the noise in the shadows. CCD needs light and lots of it and trying to push it on a dim day or late/early morning to me is not a good solution vs taking a longer exposure.
By the time you get to iso 200 on the IQ260 I feel that the shadows show quite a bit of noise and color loss, but areas that have good light can still allow for a very good clean shot, but I know I won't be able to push the shadows at all. By iso 400, there is loss in color/sat thought the file. Areas that were brightly exposed might allow for some recovery in post, but the loss in details can't be pulled back.
Of course moving to sensor plus changes all of this un to around iso 800. But the loss in resolution makes this something I rarely do.
I would love to see more information on this subject.
Paul Caldwell