In truth I don't remember how that all played out in regards to dynamic range, but what the Gemini bought to the table (and CD primarily promoted) was uncompressed 4:4:4 higher bit-depth DPX files. This was original "slog", not "slog2" of course, and undoubtedly there's a 2 hour plus long Alister Chapman video somewhere about the actual dynamic range of the camera internal vs external dual link recording.
Sony's Peter Crithary did co-op this video with CD's blessing, hacked out any reference to CD or the Gemini and played it in their NAB booth as a sensor/slog tech aid.
I think you described it better than i could have!
Will just post this inaccurate graph here, perhaps for the lurkers, if there are any anymore, as at least it shows how many cinema cameras do iso. Canon seems to add gain along the way, so, i believe Canon is a type of “true” iso, and this graph does not apply to Canon C. Also, dual iso has become popular, and those cameras have two “base” iso settings. But within each base iso, they shift iso using pushing and pulling.
However, Alexa and most Red’s tend to do iso like this:
Iso800 - cleaner shadows, “thicker” mids, but less highlight roll off.
Iso1600 - noiser shadows, “thinner” mid tone tonality, longer highlight roll off
The clipping points aren’t changing, just where the operator chooses to land the exposure. Iso 1600 boosts the brightness of the mids and allows for more highlight rolloff. Or pulling down to iso 400 cleans up the shadows but leaves less elbow room in the highlights making a “video” look up top sometimes.
There is a good interview with Rob Maclachlan who DP’d the ‘Red Wedding’ episode for the HBO series “Game of Thrones”. In the interview, he says that they shot the wedding by candlelight at iso1600, which not only gave them an extra stop to work with, but extended the highlight range keeping the candle flames from clipping.
When lowering the iso, the operator is using less of the shadows and working their back against the highlight clipping point. Whereas raising the iso, the operator it lifting the shadows and exploring the murky waters of noise, which tends to have more wiggle room for a stop or two. But highlights are that impassable barrier, there is not wiggle room with highlight clipping on digital, unless you have a lot of dynamic range for a film like infinite roll off. Wally Pfister over exposed the film in shooting the Joker interrogation scene in the Dark Knight by like 5 stops and they pulled it back down.
Same concept, different curve.
