Built-in color profile vs Log+LUT

[Nicolas POISSON]

Hello,

 

My first post here. I practice video as an enthusiast. I own a Fuji X-T30 that I use for stills. I am learning how to use it the best way for video. I am trying to understand the path of image processing within the camera and in the editing software.

My concern is about comparing those two methods:

- use built-in film simulation, like “Eterna” (low contrast, low saturation, aimed at giving a cinematic look out-of-the-box)

- use “log” profile, then apply the “log to Eterna” LUT provided by Fuji in an editing software.

 

This video is comparing those two paths: 

 

 

The author of the video, as well as most people in the comments, seem to agree that the “log+LUT” path is way better, with 2 stops increased dynamic range and so on. I don’t. I prefer the “built-in Eterna” path. I prefer the color (less greenish). I find it retains more details in the skin. But most above all, if the “log+LUT” path seems to retain a little bit more details in the highlight, it looses ten times that gain in the shadows.

OK, might sound 100% subjective right now. But with my limited understanding of how all this work, I do not see how a “Log+LUT” path could be better than “built-in” if one do use the same target color profile. My understanding is that either way, one apply some kind of LUT. When using the built-in film simulation, that LUT is applied to a high quality set of data: full depth (14 bit or so), no lossy compression. When recording with a log profile, one start form the exact same data, but one first reduce it, then one apply the LUT in the editing software. The reduction with log profile is more optimized in term of DR than a linear reduction, but it should be inferior to using the full set of data.

This is a different story for stills, as “raw” really is “RAW”. For stills, there is no reduction of quality before applying image processing in an external software. The overall image processing chain is the same, it is simply split at a different point between “built-in” and “external software” corrections. The trade off is not quality, it is “amount of data” vs. “ability to correct later”.

But video with log profile is not “raw”. It seems that even Blackmagic or Arri “raw” are not raw. My understanding is that “log+LUT” has no benefit if one use a LUT similar to the film simulation built in the camera. It is only useful if one plan to use custom LUTs that have no built-in equivalent.

So… what am I missing?

[David Mullen ASC]

You're somewhat incorrect in your conclusion.  Log will record more stops of dynamic range than Rec.709 gamma. So when you go to color-correct, you have more options than simply slapping a LUT on the image, you have control over more stops of information. In terms of whether the extended information is in the shadows or the highlights depends on how you expose it... but since clipping of bright areas is a telltale sign of video origination, log usually is exposed for extended highlight information but also the shadows are lifted too.  

But once you correct to display gamma, you usually have to lose some information but since it got recorded, you can use Power Windows and luminance keys to keep detail in some areas even once you go back to Rec.709 contrast. That's the difference between letting the camera apply the Rec.709 gamma or you doing it in post -- a camera can't do Power Windows, it can't do keys, all it can do are things like knee compression to retain highlights.

The main downside of log in most still cameras shooting video is the limitation of 8-bit recording. 8-bits isn't enough to avoid banding artifacts of smooth gradients so the lower-contrast image from log gamma tends to increase this problem. Heavy compression and low color-subsampling like 4:1:1 or 4:2:0 doesn't help either when you color-correct, which are all the arguments for not using log. However, if you can record 10-bit at 4:2:2 at least, then you might as well use log if you have access to a decent color-correction system and want to spend the time to color-correct, because you will get better results in terms of being more film-like in your highlights with log.

 

[Nicolas POISSON]

Thanks David.

Indeed, I was not thinking of the benefit of Log in general, but the comparison made in that very video above. If I understand correctly, it is basically recording log, then slapping the LUT provided by Fuji, and voilà. So if one just use a LUT that aims at doing the same thing as a built-in profile, is there still a benefit ?

And, by the way, I read plenty of people recording Log with this camera, then telling "I used the LUT provided by Fuji as a base". So if one record log, say 10 bit 422, then use Fuji's "Eterna" LUT in Davinci Resolve (without modifications), then add other steps of colour correction, is there a benefit over recording directly with that profile in the camera ?

[Stephen Sanchez]

Is there a benefit? Yes. Use log so you can play with the image in post.

But. Like David said, understand that your 8-bit video will record only so much extra dynamic range in log. If you try to grade it in to much extreme, you'll notice banding artifacts.

[David Mullen ASC]

Again a camera can’t use power windows or Luma keys to hold detail in the brightest areas so baking in a display gamma LUT in camera usually means some lost highlight detail.