New Log curve and colour space on the PMW-F5 and F55, SLog3 and SGamut3.cine

So, Sony have added a second log curve and a new colour space to the PMW-F5 and F55 cameras. This new curve and colour space does not replace the exisiting SGamut or SLog2 gamma curve. It gives a new option, but what does this new option bring to the party?

Well, first off I’m still waiting on the official white paper from Sony, so I’m making a lot of assumptions here based on what I have been able to work out for myself about this new curve based on tests with the camera and plotting the gamma curve and gamut for myself. The new gamma curve is very close to the Cineon gamma curve and the colour space is very close to the P3 DCI standard.

Looking at the colour space SGamut3.cine is very close to SGamut. So if you want the biggest possible colour gamut you should probably continue to use SGamut. But SGamut is such a large colour space that it really needs special and careful handling in post production to get the most from it. I think we all know that when you shoot with a gamma curve with a very big dynamic range and look at it on a conventional monitor it looks very flat and lacks contrast. Well the same happens with a colour gamut. Look at SGamut material on a conventional monitor and it lacks colour contrast and looks washed out. The temptation is to simply crank up the saturation to compensate, but this is far from ideal and can result in some undesirable colour shifts. What you really need to do is use a LUT to convert the SGamut colour space into the display range you are working to, but this requires careful fine tuning to match the colour space you are working in. For example you will want different LUT’s when outputting for REc-709 video or for when outputting for a P3 cinema compliant DCP.

So it appears that to try to make things a little simpler for colourists, Sony have added the SGamut3.cine colour space. This is still a very big colour space, but now very close to the industry standard DCI P3 colour space used for digital cinema post production and presentation. Looking at the SGamut3.cine images in Resolve the colours look washed out, as expected but appear to be very true to life. So any LUT’s used will have to do less work to bring your colours accurately into your chosen output colour space. By starting off with this new colour space it looks like it will be easier for colourists to quickly manipulate the image without needing complex LUT’s. It also appears closer to Rec-709, so again it should be a little easier and quicker to work with in video post production. Do remember though, if you really want the very best from the camera, SGamut does offers the biggest range, but unless you handle it correctly you may not be getting the most out of it.

So what about the new SLog3 gamma curve? Being very close to the Cineon gamma curve, SLog3 mimics film a little more closely than SLog2 (Cineon was designed to mimic film, SLog2 is designed to maximise data recorded from a video sensor). This means that SLog3 has a density response similar to film. Which in turn means that your exposure will be brighter than SLog2. Nominal Middle Grey with SLog3 is somewhere around 40% and 90% white is somewhere around 61% (awaiting for official white paper for confirmation, but my own plots should be within a couple of percent). The peak recording level with SLog3 is 94%, it does not go above 94% as does SLog2 which can go all the way up to 108%. This lower peak recording level probably comes from the fact that Cineon allows for an even greater dynamic range than 14 stops, but 94% represents a film stock with a 14 stop range. A lower maximum recording value does not mean less dynamic range, just less data being used to record the total range.

What this means is that SLog3 is allocating more data to the mid to low range than SLog2, so shadows and darker mid tones will have slightly more levels per stop than SLog2. Also mid tones will look brighter as the exposure is slightly brighter. But this comes at the expense of the range from 90% white and above being recorded with less data. Brighter Slog3 skin tones will have very slightly less data levels than Slog2, while darker skin tones will have a little more data. Slog3 will probably be easier for most people to expose correctly as it is a bit brighter overall, but will be a little less forgiving of any over exposure compared to SLog2.

In post production as the curve is so close to Cineon you will be able to use almost any Cineon compatible LUT’s or Looks. It’s also very, very close to Arri’s Log-C curve so LUT’s designed for Log-C will work very well with SLog3 and Sgamut3.cine making it much easier for many colourists to transition from cameras like the Alexa or a film based workflow to material from the F5/F55.

In summary the new Slog3/SGamut3.cine combination will be a little easier to expose by eye or with a light meter as the density profile is similar to film. However it will be a little less forgiving of any over exposure. In post it may be a little easier to work with as it is so similar to Cineon, Log-C and P3, the new colour range possibly providing a better and easier to manipulate “out of the box” image. However SGamut still appears to offer the largest colour gamut and SLog2 offers a little better over exposure latitude. SLog3 has a little better under exposure latitude.

3 thoughts on “New Log curve and colour space on the PMW-F5 and F55, SLog3 and SGamut3.cine”

  1. Thanks for this write up on SLog3, explains allot about where Sony is heading with the F series cameras. I did a test today running some CineEI SLog 3 footage through Resolve using the Arri Alexa LogC LUT to Rec709, looked great. Could you post your Slog3 plots on your blog compared to Slog and Slog2? Thanks again very informative.

  2. Thanks for the informative articles as always.

    I have a relatively straight forward green screen shoot coming up soon, and we are using the F55. Post has asked for 1080 HD, but that’s about it on specifications. Originally I was toying with the idea of shooting 4K S-LOG2, but now my thinking has shifted to shooting on a normal or hyper gamma, XAVC 4:2:2, since we’ll be in a very tightly controlled lighting scenario and I don’t want to reinvent the wheel for no benefit.

    My question is this: will shooting the greenscreen on slog3, xavc, 4:2:2 be of any advantage to the post people? I’d love to give them as much data as possible to push/pull the footage into the comp as they need, and the best looking skin tones possible. Any input is appreciated!

    1. If you can I would shoot SSTP RGB. This is fantastic for green screen. Slog is OK for greenscreen, but if you don’t have the budget for log grading the hypergammas work well.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.