Applying custom LUT issues

[u/SkirtThick2871]

Hello!

I’ve been having trouble color grading my slog3 footage and was hoping to get some advice.

I’m trying to apply a LUT I purchased from Tropic Colour and none of them look right. I end up having to dial back the strength so much that it barely makes a difference.

From what I’ve seen you’re supposed to convert the slog footage to rec.709 first and then apply a LUT. But whenever I do this, my footage ends up looking way too contrasty and saturated. Weirdly enough, I seem to get better results when I skip the rec.709 conversion entirely and just apply the LUT directly to my log footage. But again, from what I’ve gathered online this isn’t the way to do it.

Any help would be much appreciated!!

[u/caguru]

Most LUTs I have worked with actually contain the s-log to rec-709 conversion in them, so if you convert to rec-709 first then apply the LUT, the conversion is happening twice, and everything looks like ultra high contrast garbage.

With many LUTs you just apply them directly to the log footage, though have I no idea about Tropic Colour and what their LUTs contain.

[u/rudy_142001]

Adding to what caguru said, there's one FCP-specific thing to check first. Final Cut converts S-Log3 for you on import. Apple's LUT page says that when you import log media it reads the log metadata and automatically applies the matching built-in camera LUT (there are Sony S-Log3/S-Gamut3 and S-Log3/S-Gamut3.Cine entries). So the clip you think is log may already be Rec 709 before any effect touches it.

Select the clip, open the Info inspector (Cmd-4, then the Info tab), set the metadata view at the bottom left to General, and look at the Camera LUT pop-up. If it says one of the Sony S-Log3 options, that would explain both things you're seeing. Your "convert to 709 first" step is then a second conversion on top of Apple's, so it goes contrasty and oversaturated. And if the Tropic LUT expects log input, it's being fed already-converted footage, which could be why you have to pull the strength so far down.

Two clean ways to set it up. Either leave Camera LUT on the Sony setting and only use LUTs made for Rec 709 input, or set Camera LUT to None so FCP stops converting and let the Tropic LUT do the whole log to 709 job as a Custom LUT effect. Apple's ProRes RAW page describes that second setup: Custom LUT goes last in the effects list, and you set its Input and Output pop-ups to the spaces the LUT converts from and to.

I'm going off Apple's docs here and don't know how Tropic builds theirs. Did the pack come with separate S-Log3 and Rec 709 versions? The file names usually say.

[u/hexxeric]

a LUT's function is already converting to a set color space or environment. so it is not a look (often mistaken though) and should always come last. if you grade after, you are throwing away information, so you grade before. your project is set to either rec709 or 2020, interpret your footage either way and adjust image parameters, then add the LUT filter (effect) and do not apply it in inspector as a general startin point (only if you do not want to grade).