This comes up a lot. People shoot in log, take it in to the grading suite or worse still the edit suite, try to grade it and are less than happy with the end result. Some people really struggle to make log look good.
Why is this? Well we normally view our footage on a TV, monitor or computer screen that uses a gamma curve that follows what is known as a “power law” curve. While this isn’t actually a true linear type of curve, it most certainly is not a log curve. Rec-709 is a “power law” curve.
The key thing about this when trying to understand why log can be tricky to grade is that in the real world, the world that we see, as you go up in brightness for each stop brighter you go, there is twice as much light. A power law gamma such as 709 follows this fairly closely as each brighter stop recorded uses a lot more data than the previous. But log is quite different, to save space, log uses more or less the same amount of data for each stop, with the exception of the darkest stops that have very little data anyway. So conventional gamma = much more data per brighter stop, log gamma = same data for each stop.
So time to sit down somewhere quiet before trying to follow this crude explanation. It’s not totally scientifically accurate, but I hope you will get the point and I hope you will see why trying to grade Log in a conventional edit suite might not be the best thing to try to do.
Lets consider a scene where the brightness might be represented by some values and we record this scene with a convention gamma curve. The values recorded might go something like this, each additional stop being double the previous:
CONVENTIONAL RECORDING: 1 – 2 – 4 – 8 – 16
Then in post production we decide it’s a bit dark so we increase the gain by a factor of two to make the image brighter, the output becomes:
CONVENTIONAL AFTER 2x GAIN: 2 – 4 – 8 – 16 – 32
Notice that the number sequence uses the same numbers but they get bigger, doubling for each stop. In an image this would equate to a brighter picture with the same contrast.
Now lets consider recording in log. Log uses the same amount of data per stop, so the recorded levels for exactly the same scene would be something like this:
LOG RECORDING (“2″ for each stop): 1 – 2 – 4 – 6 – 8.
If in post production if we add a factor of two gain adjustment we will get the same brightness as our uncorrected conventional recording, both reach 16, but look at the middle numbers they are different, the CONTRAST will be different.
LOG AFTER 2x GAIN: 2 – 4 – 8 – 12 – 16.
It gets even worse if we want to make the log footage as bright as the corrected conventional footage. To make the log image equally bright to the corrected conventional footage we have to use 4x gain. Then we get:
LOG AFTER 4x GAIN: 4 – 8 – 16 – 24 – 32
So now we have the same final brightness for both the corrected conventional and corrected log footage but the contrast is very different. The darks and mids from the log have become brighter than they should be, compare this to the conventional after 2x gain. The contrast has changed. This is the problem with log. Applying simple gain adjustments to log footage results in both a contrast and brightness change.
So when grading log footage you will typically need to make separate corrections to the low middle and high range. You want a lift control to adjust the blacks and deep shadows, a mid point level shift for the mid range and a high end level shift. You don’t want to use gain as not only will it make the picture brighter and darker but it will also make it more or less contrasty.
One way to grade log is to use a curve tool to alter the shape of the gamma curve, pulling down the blacks while stretching out the whites. In DaVinci Resolve you have a set of log grading color wheels as well as the conventional primary color wheels. Another way to grade log is to apply a LUT to it and then grade in more conventional 709 space, although arguably any grading is best done prior to the LUT.
Possibly the best way is to use ACES. The Academy of Motion Pictures workflow takes your footage, whether log or linear and converts it to true linear within the software. Then all corrections take place in linear space where it is much more intuitive before finally be output from ACES with a film curve applied.