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It appears the issue with Slog2 and S-gamut with the A7S is related to the nonlinear S-gamut Color Mode. My current work-around is to use Cinema Color Mode. If there is a significant benefit to using S-gamut, it appears some kind of 3D LUT is needed to fix it. I few years ago I wrote custom software to do 3D color correction (for stereoscopic 3D images). The 3D color cube was directly displayed and could be edited, and it was also possible to render color swatch slices to check for issues with clipping (cube elements outside the unit cube space) or other mapping issues (such as color merging leading to banding/quantization). Color cube display (unedited / unit-normalized cube shown (actual distorted cube for current color correction not shown)): Color swatches (cube slices) showing range/clipping issues: It may be possible to fix the S-gamut color mode issue by creating a 3D LUT which fixes the green/yellow splotches in skintones and perhaps also the magenta splotches in shadows. Since green and magenta are color complements, it kind of has the feel from physics describing a form of instability. That is, the system is stable in a narrow range, and when outside that range it becomes unstable, going extremely green or magenta, depending on which side of the parabolic curve center the color(s) are on. [edit: hosted pics on my server]
Using picture profile 7 on the A7S uses Slog2 Gamma and S-Gamut Color Mode. This can result in green/yellow splotches on skintones after converting Slog2 into something that looks close to Rec709. WB settings in camera or after correction in post don't appear to matter: only certain shades of skintones have the color issues. While this can be fixed with qualifier color range correction tools, that's a lot of extra work. I found that changing the Color Mode to Cinema appears to fix the color issue (Pro Color Mode also looks pretty good). It's not clear how this might affect dynamic range (figure primarily related to gamma/luma), as that's a major point of using Slog2, however so far the results look pretty good.