@Vitaliy_Kiselev

The LOG-LUT workflow provided by the post production tools isn't designed to be as flexible as your mathematical manipulations would require. As you claim, any exposure mismatch between the footage and the LUT could be easily corrected by adjusting the gain of the footage before feeding it into the LUT. But that would require manual calibration of post production levels, and Panasonic's engineers wanted to avoid that complication. They instead enforce the proper V-Log-L exposure level at the time of recording, producing a turnkey workflow that requires no technical understanding of the process on the part of the user.

Panasonic's V-Log-L tone curve requires exposure calibration in order to precisely match the dynamic range of the footage to the curve of the Varicam V-Log LUT. The reason this is necessary is because V-Log-L is not purely logarithmic throughout its 12-stop dynamic range, the bottom 4 stops have a non-logarithmic rolloff down to 0% reflectance. In addition, the entire V-Log-L curve is offset with a 12.5% pedestal. The DVX200 (and presumeably the GH4R) enforces this LOG-LUT match by scaling the maximum sensor level down to 79 IRE and biasing the black level to 12.5%. Unfortunately, that discards the camera's 80-109 IRE range, leaving you with little more than 7-bit data precision when V-Log-L is encoded into 8-bit H.264 format. What's worse is that the bottom 4 stops of the 12-stop dynamic range are encoded with just 4 bits to cover all 4 stops. Such a crude level of digitization makes the effort to capture 12-stops of dynamic range futile. Bottom line: V-Log-L was designed for recording at 10-bit or better data precision; 8-bit H.264 encoding is not quite good enough to preserve shadow detail.

@shian, which Panasonic-provided Varicam V-Log LUT do you mean exactly?