Chroma Key and Greenscreen – Should I use S-Log3 or might Rec-709 be Better?

I’ve covered this before, but as this came up again in an online discussion I thought I would write about it again. For decades when I was doing a lot of corporate video work we shot greenscreen and chroma key with analoge or 8 bit, limited dynamic range, standard definition cameras and generally got great results (it was very common to use a bluescreen as blue spill doesn’t look as bad on skin tones as green). So now when we have cameras with much greater dynamic ranges and 10 bit recording is it better to shoot for greenscreen using S-Log3 (or any other log curve for that matter) or perhaps Rec-709?

Before going further I will say that there is no yes-no, right-wrong, answer to this question. I will also add that Rec-709 gets a bad rap because people don’t really understand how gamma curves/transfer functions actually work and how modern grading software is able to re-map the aquisition transfer function to almost any other transfer function. If you use a colour managed workflow in DaVinci Resolve it is very easy to take a Rec-709 recording and map it to S-Log3 so that you can apply the same grades to the 709 as you would to material originated using S-Log3. Of course the 709 recording may not have as much dynamic range as an S-Log3 recording, but it will “look” more or less the same.

Comming back to shooting greenscreen and chromakey:

S-Log3:
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Shoot using 10 bit S-log3 and you have 791 code values available (95-886) to record 14/15 stops of dynamic range. so on average across the entire curve each stop has around 55 code values. Between Middle Grey and +2 stops there are approx  155 code values – this region is important as this is where the majority of skin tones and the key background are likely to fall.

Rec-709:
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Shoot using vanilla Rec-709 and you are using 929 code values (90-1019) to record 6/7 stops so each stop has on average across the entire curve has around 125 code values. Between Middle Grey and +2 stops there are going to be around 340 code values.
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That is not an insignificant difference, it’s not far off the  difference between shooting with 10 bit or 12 bit.
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If you were to ask someone whether it is better to shoot using 10 bit or 12 bit I am quite sure the automatic answer would be 12 bit because the general concensus is – more bits is always better.
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A further consideration is that the Sony cameras operate at a lower ISO when shooting with standard gammas and as a result you will have an improved signal to noise ratio using 709 than when using S-log3 and this can also make it easier to achieve a good, clean, key.
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However you do also need to think about what it is you are shooting and how it will be used. If you are shooting greenscreen in a studio then you should have full control over your lighting and in most cases 6 or 7 stops is all you need, so Rec-709 should be able to capture everything comfortably well. If you are shooting outside with less control over the light perhaps Rec-709 won’t have sufficient range.
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If the background plates have been shot using S-Log3 then some people don’t like keying 709 into S-Log3. However a colour managed workflow can deal with this very easily. We should consider that 709 and S-Log3 in a workflow where grading is a big part should not be though of as “looks” but simply as transfer functions or maps of what brightness/saturation seen by the camera is recorded at what code value. Handle these transfer functions correctly via a colour managed workflow and both will “look” the same and both will grade the same within their respective capture limits.
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For an easy workflow you might chose to shoot the greenscreen elements using log with the same settings as the plates. There is nothing wrong with this, it works, it is a very commonly used workflow but it isn’t necessarily always going to be optimum. A lot of people will put a lot of emphasis on using raw or greater bit depths to maximise the quality of their keying, but overlook gamma choice altogether, simply because “Rec-709” is almost a dirty word these days.
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If you have more control, and want absolutely the best possible key, you might be better off using Rec-709. As you will have more data per stop which makes it easier for the keying software to identify edges and less noise. If using Rec-709 you want to chose a version of Rec-709 where you can turn off the camera’s knee as this will prevent the 709 curve from crushing the highlights which can make them difficult to grade. In a studio situation you shouldn’t need to use a heavy knee.

I suggest you experiment and test for yourself and not every situation will be the same, sometimes S-Log3 will be the right choice, other times Rec-709.
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One thought on “Chroma Key and Greenscreen – Should I use S-Log3 or might Rec-709 be Better?”

  1. Hi Alister, thank your for this insight. As an FX9 owner, when shooting greenscreen, would you recommend S-Cinetone in the same way, i.e. for the same reasons as Rec-709? Or is it a worse choice because a color managed workflow would not be able to convert it? (I assume Rec-709 and S-Log3 have known and standardized transfers, whereas S-Cinetone doesn’t).
    Best, Andreas

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