One common question I am often asked is how to use Log or CineEI in low light situations. After all in the Sony CineEI mode or with a log camera such as the Alexa that use EI the actual recording gain is fixed, so often the recorded pictures are very dark. So what should you do?
My answer in many cases is simply not to use Log or EI. It’ really important to remember that the primary reason why log recording was developed was to make it possible to record a very large dynamic range using existing recording technologies. In order to do this lots of compromises are made. The main one being allocating less recording data to each stop of dynamic range. If you’re recording using 10 bit you typically have about 970 useable code values or shades. Use a gamma curve with 6 stop range and you have about 160 shades per stop, record using a log or other extended range gamma with a 14 stop range and you have just 70 shades per stop.
Lets think about a couple of different scenes for a moment. Scene one is a daytime scene that’s nice and bright with an 8 stop dynamic range. Scene 2 is a night time scene that is fairly dark and only has a 5 stop dynamic range. What happens if we shoot using log?
With the daytime scene things are pretty straight forward. You just expose as per the manufacturers recommendation. Assuming your camera is set to log and capable of a 14 stop dynamic range, if exposed at the base, recommended exposure levels you will be using a little under 60% of your available recording range, so a fair bit of data will be going to waste. But as the scene is bright you can always open up the aperture a bit and deliberately over expose by a stop or two so that now your using around 75% of the available data. You don’t normally want to over expose too much as grading becomes trickier, but at least when you have a bright scene you can open up the aperture to expand the recording range a bit to make better use of the data.
In low light however it get’s trickier as your aperture may already be wide open. In the second dark scene it’s quite possible that the 5 stops of light coming back from the scene aren’t bright enough to allow you to expose at the recommended levels. So instead of filling the zero to 5 stop recording range you might only be filling the zero to 4 stop range. Even if you can expose correctly with a 14 stop recording bucket like log you are only using a little over 35% of the available data and that’s a terrible waste. You won’t have much data to help you separate out any noise from the desired picture information in post production and because your recorded signal is small any compression noise will be relatively big in comparison to the picture information.
This is when it’s time to abandon log and go back to a conventional gamma curve. You don’t need log when the scene only has a limited dynamic range. If you use Rec-709, which has a 6 stop range (without any knee) instead of log, at the same ISO, then now instead of recording using only 35% of the available data you will be using almost 85% of the available data and that’s going to give you much more real picture information to work with in post production. You will get a much better end result by not using log.
If you need a bit more than a 6 stop range then there are plenty of other gamma curves to choose from that sit between 709 and log. Gamma curves such as Sony’s cinegammas or hypergammas which have dynamic ranges between 10 and 11 stops.
So remember, when you don’t need a 14 stop recording range, then consider using an alternate gamma curve. It’s not just in low light but also in other controlled lighting situations such as green screen work.
