Now that you understand LUTs and gamma, let’s talk about log.
First of all, log is a very exciting thing for several reasons. It’s an efficient way to capture a lot of high quality detail and maintain a huge amount of post production flexibility. It is not designed as a visual aesthetic but rather a technical solution to the problem of a camera being able to capture a higher range of contrast than most display mediums can reproduce. It means smaller file sizes than shooting “raw” but with similar ability to be adjusted in post. If shooting to a sufficient bit depth, log footage can be white-balanced post-debayer in a good color application. This is often misunderstood.
Let’s look at Sony’s S-log 2 as an example of a log format.
First, the limitations:
S-Log is recorded in data range with black at code value 92 (in 10 bit speak; in 8 bits that’s code value 23) This is pretty standard for a lot of film log curves. S-log2 goes up to the equivalent 104 IRE . 70 shades per stop in 10-bit vs. only 15 shades per stop in 8-bit give you an idea of why color gets tricky. However both curves with the right camera could reach 109% or code value 1024 in 10 bit, AKA 255 in 8-bit. Divide numbers by 4 to get 8-bit values. You’ll end up with so little shadow detail when shooting S-log 2 that you can only push shadows down in post, never bring them up.
Typically if you are shooting with 8 bit, for example with an FS5 in UHD or an A7S, A7R etc, then I recommend you use S-Log2 with SGamut. For most other cameras that have 10 bit recording go ahead and use S-Log3 and SGamut3.cine. S-log 2 is designed to give you 6 stops over middle gray and 8 stops under for a total of around 14 stops. More than your sensor can provide in the case of these small Sony hybrid cameras. If you shoot log to a lower bit depth, you essentially have very few code values allotted to a very large section of image information. Once you stretch the image back out via a normalization, you’ll see the lack of data quickly and the image can become unusably posterized.
Exposing log is therefore a bit tricky and you have to get exposure correct, especially when shooting to these 8-bit formats!
Here’s a look at some exposure values for Sony cameras:
|Subject||Rec709 IRE||SLog2 IRE||SLog3||Cine 4|
|Middle Gray||40%||42% recommended (32% per Sony recommendation or 10-bit CV 347)||36% per spec?|
|Caucasian Skin||70%||60% recommended (closer to 50% per Sony) Keep skin between 45%–73%)||60%|
|White||90%||73% recommended (60% per Sony recommendation or 10-bit CV 582)|
Other flavors of log like S-log3 will place 18% gray at 40%.
If you’re using one of these charts rather than an 18% middle gray card, here’s a tip:
Middle gray “looks” middle gray but only reflects 18%.
The 18% gray value represented on X-Rite’s newer “Color Checker Video” chart is found in the strip just under white. It’s the gray square under the yellow one on Color Checker Passport.
If you don’t have a chart, Sony has an in camera meter which is generally pretty intelligent and oftentimes would expose better than you will. I like to keep it close to +2 when shooting log in a hurry. Be careful though, if it’s flashing that means it’s over +2 and you could have major issues not visible during shooting.
Overexposure is a bit tough because you lose accurate monitoring, even with the in-camera normalization features Sony provides. I’ve made LUTs to compensate for this, but you’ll need some sort of monitoring solution that allows you to preview with the -2 stop LUT applied.
First and foremost work under a LUT or CST. LUTs are highly misunderstood. They are not cheap looks designed by amateurs but important color and contrast transforms with a very specific technical purpose. Walter Volpatto, hero among men, says he does work with a LUT or LUT+look at the end using this approach. He grades big shows and still uses technical LUTs and look LUTs (in this case crafted by some very talented people like Steve Yedlin). Here’s his approach, and it’s pretty similar to the way most old-school colorists work:
So what if we overexposed log footage, what then?
This is a much easier fix than it used to be, but it still requires an understanding of the process. Right-click your first node and set its mode to “linear”. Now your gain wheel functions much as an exposure control on your camera would. Cool! Get your image looking exposed properly, previewing it underneath the normalizing LUT. Or, if you prefer a simpler approach, you can also set your pivot all the way down and then use the contrast adjustment to reign in the overexposure.