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S-Log3: Better or Worse for Low Light? (a7sIII)

LensMeAHand
Key Contributor

There doesn’t seem to be a consensus on this. I’ve heard you shouldn’t shoot night footage with a Log color profile because Log makes compromises to achieve a wider dynamic range, which you don’t need at night. So if that's true, it follows that you should shoot with a standard color profile to capture more information. However, I’ve worked on movies, TV shows, and commercials, and I've never seen any footage shot in anything other than Log.

I decided to shoot some test footage with my Sony a7sIII and let you decide what’s better.

Here's one shot, with four different camera settings:lex image comparison.jpg

The bottom right frame was exposed properly with no Picture Profile. The other three were shot in SLog-3 with various ISOs and then graded to match the first image.

When you zoom in, you can see differences in noise:lex red square detail.jpglex detail comparison.jpgAt ISO 8,000, the SLog-3 shot is so noisy that it's unusable. When we increase the ISO to 20,000, it's still noisier than the PP Off shot, but just barely. Even though it's clearly noisier when zoomed in on a still frame, the difference in noise isn't apparent when you playback the clips as video.

Then at ISO 64,000, the noise is just as low as the PP Off image, but at ISO 64,000, the image has been overexposed to the point that it looks slightly washed out in terms of contrast. Also, a subjective critique, but I think the colors look worse in the ISO 64,000 image. A point where I notice both these things (muted color and low contrast) is the part of the road illuminated by the headlights

Personally, I think the SLog-3 image shot at ISO 20,000 looks the best, and from this experiment, I would decide it's better to shoot in SLog-3.

So I think I'll continue to shoot in Log. But maybe I'm missing something. Does anyone have different thoughts on this? Maybe different results from different cameras? I’d love to hear other experiences.

4 REPLIES 4

julienjarry
Top Contributor

This is great and the examples you captured are awesome. 

To answer your question, for me, I shoot in slog3 because it's an essential part of my workflow. It allows me to add a blanket lut and grade across an entire project and I really like it. I love the image. Of course, noise has been a hot topic for a long time. My consensus from your tests and the fact that most of the industry captures Log no matter the scene is that we're not punching in 200% to gauge noise levels while viewing a film and a certain amount of noise is acceptable. For really important work where this conversation comes up in pre-production and healthy noise levels are important, I think being intentional about how you rate your camera at one of it's two base ISO's is what is really important. 
I think if you give your examples a big vibe and then take PP Off and try to pull that over to match the others you may see how PP Off has it's limitations.
Well done with these and it's always great to see.

I agree with everything you said. I was grading all the log footage to match the PP off shot, but if I had been grading them both to match a certain look, I would’ve had very little flexibility with the PP off.

Also, regarding noise—I would guess that when digital cameras were more primitive, there may have been some difference in noise levels when shooting with and without log, but at least with the a7sIII, I think it’s safe to say that the S-Log3 noise is a non-issue.

Not to take anything away from your testing, but any high-end production that is shooting LOG will surely be grading in Resolve Studio, or Baselight, or some other full-featured grading program where adding the necessary amount of noise reduction in post is simple and effective.  Of course, if someone is only using the free version of Resolve then I think NR is one of the features not available.  My advice is to upgrade.  Resolve was a $500K program a few years ago that couldn't do 1/10th of what it can do today.  $300 is a bargain. 

On my F55, I only ever shoot S-LOG, RAW, or X-OCN and I pretty add a touch of NR on the final node of EVERY grade.  It does help, even with "normal" footage where you probably wouldn't even think noise was an issue.  On my FX6, FS7, and A1, I only add a little NR to S-LOG when I think it is necessary.

To answer your original question, if the rest of the production is being shot in LOG, I'd certainly want to keep the low-light stuff in LOG also.

 

I think there is a bit of a common miss-conception about grading flexibility with different gammas. S-Log doesn't have any greater grading flexibility than any other gamma. However so many of the commonly used tools such as LUT's or other plugins found in most edit applications evolved as simply ways to grade log footage within the constraints of a 709 workflow so many have gained the impression that Log has more flexibility. In reality if you take log, 709 or any other gamma into a decent post production workflow such as a proper colour managed workflow like ACES and you eliminate the gamma by transforming it into a linear space then there is no difference in the grading flexibility, all will grade equally well as in essence all contain the very same image information.

Gamma curves are simply a mathematical calculation applied to the original material to make it fit in a limited recording range. They do not make an image more or less gradable and being a simple mathematical calculation they can be reversed to remove the gamma. 

What S-Log does allow for is a greater dynamic range and a larger DR might be important for many shots, but this isn't always the case. This greater range is achieved by squashing the original material using log math and this allows more stops to be stored in the same size data bucket. But, that comes at a price. S-Log3 and 14+ stops recorded into code values 95-886  means each stop on average has 56 code values (you actually have more for brighter stops but less for stops below middle grey). If you use vanilla Rec-709, with no knee (not S-Cinetone or any of the other modified versions of 709) then you will have 6 stops in code values 64 to 1023 so on average each stop will have 159 code values, almost 3 times as many as log. I'm quite sure that most reading this will already be aware of the advantages that more code values per stop brings, after all it's why we prefer 10 bit over 8 bit recordings. The difference in code values per stop between S-Log3 and vanilla Rec-709 is not that different to the difference between an 8 bit and a 10 bit recording.

However in a considerably under exposed situation where neither the log or the 709 can be fully exposed it isn't going to make a huge difference what gamma you use because the limiting factor is the actual sensitivity of the sensor and the noise it produces. To go from a certain scene brightness to a like for like final screen brightness the cumulative gain used (sensor gain + gamma gain + post gain) will be the same whatever gamma you use, so the ratio of signal to noise will be broadly the same whatever route you take and it's the noise that will likely limit how well you can grade or manipulate the material rather than the gamma choice.

But if your gamma choice allows you to fill your recording range rather than only using a small part of it then you will have more code values per stop with a narrow range gamma than a wide range one and that will provide more textures and that will grade better in just the same way as a 10 bit recording with 970 code values will grade better than an 8 bit recording with 235 values. So for some studio applications where you have control over your lighting there can be advantages for using 709.

But all of this also requires care with your workflow, simply dropping the clips into an edit applications timeline isn't going to be ideal as you will be constrained by the working gamma. You really want to use a proper grading tool with good colour management if you really want to get the best results.

Alister Chapman
Cinematographer/Producer/Trainer