You seem to have ignored the divergent clipping point for 125 ISO
I have not ignored it. It's easy to analyze the images with my raw analyzer, but it is quite laborous to create a demonstration with proofs
(which requires lots of co-ordinated screen shots). As the in-between ISOs are interesting only for JPEGs, I did not find it worthy to work a lot with that. To boot it, the ISO100-125 issue is particularly messy.
This seems to indicate that there isn't simply a multiplication going on, but something else.
In every other case (250, 500, 1000 etc.), the difference between clipping point from the full stop is merely 1%, but for 100 -> 125 ISO, the difference is 18.5%.
I'm not sure whether this discrepancy is significant
Sure, the ISO 100 - ISO 125 relationship is different from the others.
1. Please note, that one has to substract the black level from all values (it is a constant additive) before calculating any ratios. Its average is or close to 1024.
2. The ratio between the ranges of ISO200 and ISO160 close to 1.3. The ratio between the ranges of ISO125 and ISO100 is 1.2.
Both ratios are quite far from 1/3 stop, which is 1.26 . Accordingly, these ratios represent no important aspect.
3. The histograms of an ISO200 and ISO160 shots are virtually equal. Note, that the histograms are relative to the actual range of pixel values, though not exactly, for example where the value range is 16220, the histogram range is 16384.
The ISO 160 values are derived from the ISO 200 values by simply a division, it appears linear. The histograms of the mapped values are virually identical (they are useful, because the black level correction and the value range are considered in those).
4. The comparable histograms of ISO100 and ISO125 shots don't behave so nicely; the ISO 125 is more stretched. Detailed analysis indicates, that the ISO 125 is not a truely linear transformation of the ISO 100 values.
5. There is another clear indication, that the +1/3 step ISOs are derived from the full-stop ISOs: there are "holes" in the ISO 125 values: red and blue do not occur with all possible values (but their holes are not together). The following seuence represents consecutive pixel values:
RGB, RGB, GB, RG, RGB, RGB, RGB, GB, RG, RGB, RGB, GB, RG, ...
There are no holes in green; I suspect, that the two different greens would show holes, but I don't care enough to go after that.
A layered TIFF can be donwloaded with the screenshots of histograms of eight shots, pairwise fromHERE
But I do hope that you've reported the ACR bug to Adobe.
Right. I posted it on the respective Adobe forum, but no-one cared to answer it.
The problem of misrepresenting the clipping point is not new, it occurs with several cameras. However, now with the 14-bit raw files and huge pixel value ranges this is more of a problem.