The question over whether to deliberately underexpose or not with S-Log came up recently. I believe that you need to evaluate the entire scene when shooting S-Log and that the often heard “underexpose by a stop” methodology may have some issues. Here’s my take on the situation:
A couple of caveat’s first: Most of my F3 S-Log work has been in indoor situations as I have been tied to recording to various less than portable 10 bit recording solutions, so very often using a restricted contrast range. I’ve only owned S-Log for my F3’s for a short while now, so many of my earlier tests were on 3rd party cameras, some of these were beta cameras.
I have not fully tied down my workflow. I’m still investigating external recorders, everything from the Ninja, Ki-Pro, Sound Devices and of course Gemini. I’m leaning very heavily towards the Gemini as I do a lot of 3D and the Gemini LCD makes for a fantastic monitor.
Back to exposure, this is obviously going to be a slightly contentious area as there is no real “correct way to do it”. While I might not agree with pinning skin tones or anything else for that matter to one particular brightness range, that does not mean I’m right and anyone else is wrong, it is just a different approach and methodology. At the end of the day, if it works for you and gets the results you want, then that will be the way you should go, these things are not black and white, right or wrong.
A very un-scientific test that a did a while back was an eye opener for me. I was exploring the finite latitude of S-Log compared to the F3’s cinegammas. I did a couple of very quick shots, you will find them here: http://www.xdcam-user.com/2011/06/pmw-f3-s-log-and-cinegamma-quick-look/
When I filmed these two examples I was looking at dynamic range, I exposed in both cases with the bright whites of the back wall behind the girl just going into clipping so I could then see how far into the shadows I could still see useable detail. I was not concerned about getting the skin tone exposure correct. When you look at the raw S-Log it really looks pretty shocking and even I wasn’t sure how much I would recover from the highlights and the girl is a good stop overexposed. However after a very simple grade using only the colour corrector in FCP, I was able to extract a pretty good looking image and it’s amazing how much detail was actually retained in what looked like over exposed high lights. The Girl’s skin tones which I’ve measured at over 85IRE came down very nicely without any issue. A proper grade in a grading suite would I’m sure improve them still further.
What this very crude test told me was that you have incredible flexibility over where you put skin tones, you can comfortably move them up and down in post by a quite significant margin. Also seemingly overexposed S-Log highlights will contain surprisingly large amounts of fully recoverable detail. In the same test I graded the Cinegamma material to try to recover the shadow detail that was lost by due to the reduced latitude. This involved attempting to pull up the shadow areas. While this was somewhat successful, what became very apparent was the way the noise increased quite dramatically, this is something I have been aware of since I started using Cinegammas many years ago, pulling levels up will increase noise.
So… when I expose with Cinegammas (as I have done for many years) I have always been very conscious of the noticeable effect on noise that trying to lift underexposed parts of the image has. Very often in the grade the limiting factor as to how far you can push the image has been down to the noise floor and noise effects. This has mainly been with Sony EX’s which have a 54db noise floor.
Now with the F3 with have a dilemma! S-Log gives us another +1.5ish stops of dynamic range, but at the expense of a +6db increase in noise due to the +1 stop increase in sensitivity associated with S-Log.
Lets say for example that we shoot a shot with a person and we under expose the face by one stop (one stop = 6db).
If we do this with with the Cinegammas and then grade the shot bringing the face up one stop then the noise will increase by 6db from the base noise figure of 63db giving a final noise figure of approx 57db (in the case of signal to noise, a lower number is worse).
If we do this with S-Log and then grade the shot bringing up the face by one stop then the noise will increase by 6db from the base of 57db giving a final noise figure of approx 51db.
So the S-Log image becomes twice as noisy as the cinegamma material and therefore depending on the footage, it is quite possible that you would actually be able to push mid ranges and shadows further with Cinegammas than S-log in an underexposed situation due to noise issues. The S-Log and Cinegamma curves are almost identical up to over 50IRE, so latitude performance under 50IRE is essentially the same. See the charts on this page: http://www.xdcam-user.com/2011/05/s-log-a-further-in-depth-look/
If I get some time at IBC I might see if I can set up some tests to show this in practice.
Now given that I have seen for myself how with S-Log skin tones can be pushed down just as much as up in post, I tend to try to evaluate the entire scene and consider how it will be treated in post before choosing how to expose. In particular I don’t want to expose so that the entire scene will end up being lifted by a significant amount, as noise will become a concern. This isn’t always going to be possible as there are many shots where highlights have to be protected, but I don’t believe that you have to set skins etc at any particular narrow brightness range, I tend to let skin ride somewhere between 45IRE and 70IRE depending on the overall scene.
If I can fit the contrast range of the scene into the 11.5 stops of a cinegamma then I will often use the cinegammas over S-Log because of the noise improvement. S-Log comes into it’s own where you have an extreme contrast range that needs to be captured. However at the end of the day you do still have to remember that the end display device is unlikely to be able to display more than 7 stops with any accuracy!
One tool I have found very useful is the BlackMagic HDLink box. I often use this to connect to a monitor as it has the ability to apply LUT’s very quickly. If you have a PC connected to the HDLink you can go in an modify the LUT curve in real time and in effect do an on-set grade. The HDLink is only $499 USD.