Why Does S-Log Recorded Internally Look Different To S-Log Recorded On An External Recorder?

I have written about this many times before, but I’ll try to be a bit more concise here. So – You have recorded S-Log2 or S-Log3 on your Sony camera and at the same time recorded on an external ProRes Recorder such as an Atomos, Blackmagic or other ProRes recorder. But the pictures look different and they don’t grade in the same way. It’s a common problem. Often the external recording will look more contrasty and when you add a LUT the blacks and shadow areas come out very differently. Video signals can be recorded using a several different data ranges. S-Log2 and S-Log3 signals are always Data Range.  When you record in the camera the cameras adds information to the recording called metadata that tells your editing or grading software that the material is Data Range. This way the edit and grading software knows how to correctly handle the footage and how to apply any LUT’s. However when you record to an external recorder the external recorder doesn’t have this extra metadata. So the recorder will record the Data Range signal that comes from the camera but it doesn’t add the metadata. The ProRes codec is normally used for Legal Range video and by default, unless there is metadata that says otherwise, edit and grading software will assume any ProRes recordings to be Legal Range. So what happens is that your edit software takes the file, assumes it’s Legal Range and handles it as a Legal Range file when in fact the data in the file is Data Range. This results in the recording levels being transposed into incorrect levels for processing. So when you add a LUT it will look wrong, perhaps with very dark shadows or very bright over exposed looking highlights. It can also limit how much you can grade the footage. What Can We Do About It? Premiere CC. You don’t need to do anything in Premiere for the internal .mp4 or MXF recordings. They are handled correctly but Premiere isn’t handling the ProRes files correctly. My approach for this has always been to use the legacy fast color corrector filter to transform the input range to the required output range. If you apply the fast color corrector filter to a clip you can use the input and output level sliders to set the input and output range. In this case we need to set the output black level to CV16 (as that is legal range black) and we need to set output white to CV235 to match legal range white. If you do this you will then see that the external recording appears to have almost exactly the same values as the internal recording. However there is some non-linearity in the transform, it’s not quite perfect.
Screenshot-2019-03-01-at-11.04.04 Why Does S-Log Recorded Internally Look Different To S-Log Recorded On An External Recorder?
Using the legacy “fast color corrector” filter to transform the external recording to the correct range within Premiere.
Now when you apply a LUT the picture and the levels are more or less what you would expect and almost identical to the internal recordings. I say almost because there is a slight hue shift. I don’t know where the hue shift comes from. In Resolve the internal and external recordings look pretty much identical and there is no hue shift. In Premiere they are not quite the same. The hue is slightly different and I don’t know why. My recommendation – use Resolve, it’s so much better for anything that needs any form of grading or color correction. DaVinci Resolve: It’s very easy to tell Resolve to treat the clips as Data Range recordings. In the media bin, right click on the clip and under “clip attributes” change the input range from “auto” to “full”. If you don’t do this DaVinci Resolve will assume the ProRes file to be legal range and it will scale the clip incorrectly in the same way as Premiere does. But if you tell Resolve the clip is full range then it is handled correctly.

4 thoughts on “Why Does S-Log Recorded Internally Look Different To S-Log Recorded On An External Recorder?”

  1. I have been using the Sony HXR-IFR5 and the R5 recorder with my PXW-FS5 to get 12 bit linear raw and I am finding that the images are getting very close to 4K raw from my F5. This setup has given me the ability to get raw with my FS5 on a Ronin S tethered to the recorder. The footage is better than the FS5 Shogun Inferno that I had been using. Does this make sense or is it some type of placebo effect.

    1. The Sony raw codec in the R5 is very good but raw is raw, you are recording the same data whether using the R5 or any other recorder. Any differences are most likely down to how the files are handled in by your post production workflow rather than any significant difference in the recordings themselves.

  2. I was just looking at my FX9 and comparing levels from SLog3 through the s709 LUT I have loaded into my Odyssey 7Q with the output of the camera going into the same Odyssey using the s709 LUT from the camera . I have the Odyssey LUT set for extended but it is reading about a 1/3 stop hotter than the same LUT coming out of the camera. If the camera is set to an EI 1/3 higher, then the LUT’s match. Why is this the case?

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