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A camera with such great dynamic range performance suggests it's probably fairly ISO-invariant, but is it? It's ISO-invariant in exactly the way it should be, but not so in the ways it be. You can think of this circuitry as amplifying the signal at the pixel level more than at lower ISOs, at the cost of higher tones, to protect it from any downstream noise.
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We've been singing this tune since 2014 when we designed our ISO-invariance test, and it's even more relevant today with dual-gain architectures.
ACR understands digital 'push' tags and you can brighten the image preview (and JPEG) as necessary.
Impressive, though keep in mind again that the overall image quality improvement of an ISO 64 file from a D850 is due to total captured light (and it's all about total captured light, which you can read about here).
Independently, our friend Bill Claff has tested the a7R III and also shows a similar 0.3 EV improvement over the Mark II (you can see the dynamic range numbers by clicking on the relevant camera in the legend at the upper right).
He also shows the slight advantage of the Nikon D850 over the a7R III, which comes in at 13.7 EV vs. to high ISO performance (we have comparisons coming showing parity between high ISO a7R III and a9 performance).
Now, Sony, if you could please offer us visually lossless compressed Raw so we don't have to deal with 80MB files (and longer write times and fewer images per card) for no reason, I'm sure we would all be happy... The a7R III, like many Sony predecessors, has a separate higher conversion gain (HCG) circuit at the pixel.
At ISO 100, the a7R III dynamic range tones in your image benefit from the higher signal:noise ratio—even midtones and brighter tones will be more amenable to post-processing and sharpening thanks to being more 'clean' and less noisy to begin with.
The D850 is able to tolerate as much total light as the medium format Fujifilm GFX 50S, as we showed here.
This is not to single out Sony: Nikon, Olympus and Panasonic are just as easy to blame, if not Canon of late after having modernized its sensor architecture to catch up with the rest.