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I learned this technique some 6 or 7 years ago in some short Photoshop course and totally forgot about it. There are different combinations of CMYK percentages that represent common variations of proper skintones from various races. Of course, no skintone is exactly the same so this is used as a guideline and you adjust the percentages just a little bit for your actual subject. Then you can throw that corrected still into your editing suite and use it as a reference. Here is tutorial link: http://www.graphicconnectionkc.com/skin-tone-correction.html At the bottom of that page are most common skintones with CMYK percentages.

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]

GH4 + Voigtlander 25mm @ F.95, very low light. 1920x1080: '>