It sounds interesting but I don't think I'm picking up what you're putting down... What do you mean when you say the camera (presumably back) doesn't have ISO above 100 and that the higher settings are simply "underexposures". How does this differ from a camera that, for lack of a better vocabulary, has a "true" ISO 200 (or higher).
Digital sensors do not have different light sensitivities; they capture photons and "count" them. Unfortunately, this "counting" is not an accurate process. I tried to describe this in
Post #55. In short,
ISO gain means, that the sensor tries to get more detailed data from the same electric charge, though this is inherently less accurate, i.e. more noisy.
When you underexpose a shot and "correct" it in raw processing, for example by the "Exposure" slider of ACR, the raw processor simply multiplies the original raw values. +1 EV causes a multiplication by two. The noise (irregularity) gets multiplied by two as well; thus, areas, which were too dark, now appear brighter - but noisier.
The ISO gain is noisy as well, but to a lesser degree. When increasing the ISO by one stop, the pixel values get
roughly doubled, but the noise is less than doubled. With higher and higher ISO, the "noise gain" increases, to a point, where the ISO gain is not better than simply multiplying all pixel values. Some cameras offer even higher ISO settings, but they simulate the effect by multiplying the values in-camera, just like in raw processing these are "fake ISOs".
it is a phenomena of ALL digital sensors and cameras, not just MF DB's; The Leica M8 does this, the Nikon D3 does this and the Canon 1Ds3 does this
It is not a common phenomenon for all digital sensors, not even for MFDBs. I know definitively, that the Phase One P45 Plus does have real ISO gain.
Let's see the difference between the ISO effects of these two cameras.
First, two captures from the P45+; the first is with ISO 400, shutter 1/80s, aperture f/5; the second with ISO 800, shutter 1/80s, aperture f/7.1.
I selected a small area (marked by the orangy rectangle), which is uniformly colored and smooth, thus the noise can be easily measured in term of standard deviation per color.
The red, green and blue pixel averages in the seelcted area are marked by yellow, the standard deviations by magenta.
The average raw pixel values are very close despite the one stop lower exposure of the ISO 800 shot, i.e. the values are roughly doubled (the f/7.1 shot captured only half the photons compared to the f/5 shot). However,
the noise is much less than doubled.
Now let's see the comparable P25+ shots. The first is ISO 400, 1/8s, f/13, the second is ISO 800, 1/15s, f/13. I applied +3 EV respectively +4 EV brightness adjustment in the display; this does not affect the raw pixel values, only the RGB.
As the exposure was halved without ISO gain, the second shot shows much lower pixel values (somewhat less than the half of those in the ISO 400 shot). However,
the noise is much more than half of that in the ISO 800 shot due to the low exposure (this shot is almost 3 EV from the right edge, as shown by the histogram above).
When the pixel values get doubled in raw processing, the noise gets doubled as well, and that will be much more, than it was in the ISO 400 shot.
Please ignore the appearance of noise on these images, they are GIFs. This
layered TIFF file contains the sceen captures without the deterrioration caused by GIF.
An
important note: one might think based on these shots, that the noise of the P25+ at ISO 400 is less, than that of the P45+. This comparison is not valid; the P45+ shot can be compared only to the other P25+ shot, because the P45+ ISO 400 shot is already with two stops ISO gain!
the Nikon D3 does this and the Canon 1Ds3 does this
All present DSLRs I have analyzed (two-three dozens) have real ISO gains, but many, perhaps most of them support even higher settings, and
those settings are numerical dervatives, done in-camera, which is useful for JPEG only. For example the Nikon D3's highest true ISO is 6400.
the latest generation of MF DB offers a significantly better "noise" signature than any DSLR at similar ISO, at least in my opinion and for my uses
I can hardly debate your uses, but from the present shots is clear: the P25+ has a very good noise characteristic at ISO 100, but the lack of real ISO gain is visible when underexposed.
The consequence of all the above is, that "higher ISO" should be used with the P25+
only when the exposure can not be increased at all, not as a substitute for higher exposure.