what you mean when you say Thierry's camera (mine too) doesn't have ISO 800.
It means, that when you select ISO 200, the metering will yield one stop lower exposure than with ISO 100, just like with a -1 EV exposure bias. Additionally, the camera records, that your intention was to use ISO 200, and the raw processor will apply a +1 EV correction (+2 EV with ISO 400, +3 EV with ISO 800).
This is true re the eMotion 54. I am not sure regarding the e75: I see that it does not have ISO 800, but it is possible, that it has ISO 400. I don't have comparable raw files to analyze that (I need the same scenery, same illumination, with ISO 50, 100, 200, etc., either with constant exposure, or with 1 EV reduced at each ISO increase).
However, Thierry's shot is exactly three stops lower exposed than the very right edge; this mad me think that the exposure had been calculated three stops lower than "normal". Otherwise he would have had to underexpose by two stops - not very probable.
Befor you get scared: this is not unique. The Phase One P25+, and apparently the P30 too don't have different ISO gains; the P45+ does. Please note the difference between the ISO
setting and the ISO
gain.
This is all right, because these cameras have a huge dynamic range and deliver very clean pixels. However, there is a downside of this, cased not by the camera but by Brumbaer's and eXposure, therefor I suggest not to use fictional ISOs.
Keep in eyes, that there is no ISO gain, i.e. the image data is the same as it would be with any other ISO setting if the exposure were the same. You use higher ISO in circumstances, when you can not expose high enough with the "standard" ISO, whatever that is. So, now ypu select ISO 400 and expose two stops lower, than you would do with ISO 100. However, there may be highlights in the scenery; these would be blown with real ISO gain, but not with the fictional ISO. This means, that you can "recover" (this is incorrect here) areas, which
appear blown. However, as the automatic compensation for the underexposure is realized, everything higher than the fictional limit is regarded by ACR as "blown out"; you can gain it back by the exposure or by the "recovery" slider, but if you don't know this, you get fooled.
I don't know, how it works with Aperture, but what I see in the DNG file generated by Brumbaer's and eXposure indicates, that the same will happen there.
Capture One may treat the original raw files differently, more sensibly, I don't know.