Is 8 bit OK for S-Log

[alisterchapman]

My opinion is that while 8 bit, 422 can be used for S-Log, it is not something I would recommend. I'd rather use a cinegamma with 8 bit recording. 10 bit 422 S-log is another matter altogether, this is well worth using and works very well indeed. It's not so much whether you use 444, 422 or maybe even 420, but the number of bits that you use to record your output.

What you have to consider is this. With 8 bit, you have 240 shades of grey from black to super white. of the 256 bits available, 16 are used for sync, white is at 235 and super white 256 so black to 100% white is only 219. With Rec-709, standard gamma, on an F3 you get about an 8 stop range, so each stop of exposure has about 30 shades of grey. When you go to S-Log, you now have around 13 stops of DR, so now each stop only has 18 shades of grey. Potentially using 8 bit for S-Log, before you even start to grade, your image will be seriously degraded if you have any flat or near flat surfaces like walls or the sky in your scene.

Now think about how you expose S-Log. Mid grey sits at 38% when you shoot. If you then grade this to Rec-709 for display on a normal TV then you are going to stretch the lower end of your image by approx 30%, so when you stretch you 18 steps of S-Log grey to get to Rec-709 you then end up with the equivalent of only around 12 shades of grey for each stop, that's less than half of what you would have if you had originally shot using Rec-709. I'm sure most of us have at some point seen banding on walls or the sky with standard gammas and 8 bit, just imagine what might happen if you effectively halve the number of grey shades you have.

By way of a contrast, just consider that 10 bit has 956 grey shades from black to super white. the first 64 bits are used for sync and other data, 100% white is bit 940 and super white 1019. So when shooting S-Log using 10 bit you have about 73 grey shades per stop, a four fold improvement over 8 bit S-Log so even after shooting S-Log and grading to Rec-709 there are still almost twice as many grey shades than if you had originally shot at 8 bit Rec-709.

This is a bit of an over simplification as during the grading process, if your workflow is fully optimised you would be grading from 8 bit to 10 bit and there are ways of taking your original 8 bit master and extrapolating additional grey shades from that signal through smoothing or other calculations. But the reality is that 8 bits for a 13 stop dynamic range is really not enough.

The whole reason for S-Log is to give us a way to take the 14ish stop range of a typical linear 12 bit camera sensor and squeeze as much of that signal as possible into a signal that remains useable and will pass through existing editing and post production workflows without the need for extensive processing such as de-bayering or RAW conversion. So our signal which starts at 12 bits has already been heavily processed to get it from 12 bits to 10. Going from 10 bit down to 8 is a step too far IMHO.

[Paul Ream]

That all makes perfect sense - thank you.

Can I ask a silly but sort of related question:

Assuming you've got a 10bit 422 recorder but your clients don't have the time or ability to grade S-Log, is there any difference to the available dynamic range between simply recoding a normal picture profile (say REC-709) and alternatively using S-Log but recording the REC-709 LUT output?

Logic says they should be close to identical in DR but I'm not sure if they are? This question may get more relevant if the S-Log included in firmware 1.4 can be graded in camera just like any other picture profile. It might make for some interesting looks!

[alisterchapman]

V1.4 firmware will allow you to modify the look of an S-Log profile.

Depends on the LUT as to how much DR you get, but they all have slightly less DR than raw S-Log and are much harder to grade if you don't like the look. In addition as they are 1D LUT's they won't add any saturation so the likely hood is that some grading will still be needed.

[Paul Ream]

Thank you for the reply. It'll be interesting to see how well the S-Log profile can be graded in camera. Exciting days.

[starcentral]

Here are some shots I did recently using 8-bit for S-LOG and I'm quite pleased with the results. This is not the final grade, as the final will probably be toned down somewhat. Anyone want to guess if this was 420, 422 or 444?

[alisterchapman]

If that's the before and after, then your S-Log is well over exposed. White should only be 68%. If it had been exposed correctly the highlights on the guys head would have graded out much better and the skin tones would look more natural. Looking at the shot, you didn't need to use S-Log anyway. REC-709 would have coped with the range in the shot and as your recording 8 bit, given you more data per stop which would grade better. The shirt over exposure is showing the typical effects of overexposed S-Log where it burns out with only the slightest provocation in the grade due to the ever increasing amounts of compression that occur above 60%

[truelife]

I recently recorded S-log to both SxS and (as a test) a Hyperdeck Shuttle 2. I knew that the Hyperdeck would only give me DnxHd files, but I've successfully used them in FCPX before so it seemed to be a non issue. The footage from the Hyperdeck looks great. Flat as expected. However, the SxS footage looks like it has a LUT applied even though LUT was turned off in the camera. Am I missing something? Or was there a gamma shift in the DnxHD files? The SxS footage definitely doesn't have that flat look that S-log gives. Any thoughts?

[alisterchapman]

The internal SxS files and files recorded externally should really look the same. I don't know why they look different unless you are getting a gamma shift somewhere.