Using The S-Log3 LUT To Bake In The EI

Many people wish to bake in the cameras Exposure Index settings when shooting using CineEI in order to avoid having to make an exposure correction in post production (given that the cameras are ISO invariant when shooting Log in reality it makes vey little difference whether you add gain in the camera or in post production – gain is gain). On cameras such as the FS7, FX6 or FX9 one way to do this is by baking in the built in S-Log3 LUT.  To avoid confusion – that is using the CineEI mode with the “S-Log3” LUT enabled and in the LUT settings “Internal recording” set to ON so that you are recording the “S-Log3” LUT.

While this will bake in the EI change, this technique comes with many issues. For a start, just as when you use S-Log3 in custom mode and alter the ISO, whenever you move away from the cameras base ISO you loose dynamic range. When you bake in a LUT and change the EI, you are in effect changing the ISO and there will be a corresponding loss of dynamic range. When you bake in a LUT this loss of dynamic range is exacerbated by a reduced or altered recording range.

At lower EI’s the available recording range shrinks as the LUT is made darker and at the same time upper recoding level of the LUT is reduced. At 200 EI the recording range only gets to around 78%. At the bottom end the shadows are crushed and shadow information lost by the range reduction. This then causes a post production issue because LUT’s designed for the normal S-Log3 input range of 0-94% will now be applied to recordings with a much reduced range and after application of a LUT in post the final output won’t get to 100% without further complex grading where the image will need to be stretched more than normal and this degrades quality.

At high EI’s the LUT becomes brighter but the clip point remains the same.  So for each stop you go up, 1 stop of highlights just disappears beyond the LUT’s hard clip point and can’t be ever recorded. Again in post this can cause issues because when you apply a normal S-Log3 LUT the heavy clipping in the recording causes the highlights to look very heavily clipped (because they are). Again, for the best results you will need to grade your footage to allow for this.

So, in practice the idea of baking in the S-Log3 LUT to eliminate the need to do any post production corrections doesn’t work because the addition of the S-Log3 LUT introduces new limitations that will need to be corrected if you want good looking images. Plus adding the S-Log3 LUT in camera and then adding another LUT on top in post is never going to deliver the best results due to the way LUT’s divide the image into brightness zones.

And – if you are baking in the S-Log3 LUT, then really this is no longer EI as there is now no longer an offset between the exposure and the recording, you are simply recording at a higher/lower ISO.

10 thoughts on “Using The S-Log3 LUT To Bake In The EI”

  1. Hello Alister,
    “adding a LUT in camera and then adding another LUT on top in post…”

    But I assumed that the point of burning in a LUT in-camera (LC709typeA) is on the expectation that a LUT would *not* be applied in post… in other words, treat that clip as you would a slightly flat rec709 clip, that needs just a small tweak to make a pleasing picture.

    Or am I missing something?

    1. This is specifically about baking in the “S-Log3” LUT to record S-Log3 at a different ISO to the base ISO.

  2. If youre baking in a LUT just shoot at the high or low base iso. Right? Always had success with that. Very helpful for quick tv turnarounds.

    1. This isn’t abut when you bake in a LUT to determine the final look of the image. This is about baking in the “S-Log3” LUT so that the recording is still S-Log3 but now at a different ISO to the base ISO.

      1. is 65x supported into the F55, FS7 and FX3/FX6/FX9 ? Would it be a better choice since you have more color information?

        1. No, they only support 33x. I don’t know of any camera that actually processes 65x LUTs internally. Although you can load a 65x LUT into a Venice the LUT is converted to 33x for image processing. It would be very impressive to see a camera handle the processing needed for 65x without a lot of heat, power and image lag.

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