If shooting in low light don’t use Log!

XDCAM-sliding-system-v2 If shooting in low light don't use Log!

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.

23 thoughts on “If shooting in low light don’t use Log!”

  1. Alister, your posts so often help me to understand this stuff at a deeper level. Thank you so much for taking the time to do this! It’s much appreciated.

  2. What about burning in a LUT in low light.. or are they all fairly wide DR compared to REC 709.. and so still wasting data..

    And thanks for the continued informative postings .. I was getting worried for a while there !

  3. Yes, you could burn in a narrow range LUT such as 709(800). You could make a LUT specifically for low light applications.

  4. Great post!

    I have a question regarding the differences between CINE EI mode and CUSTOM MODE. What is the bit-depth of the SLOG-3 CINE EI image before applying the luts and recording? 10 bit? 12bit?
    And in CUSTOM mode, how do I match an image to the SLOG-3.cine color space, so I know that my primaries are in the same place. I have had some bad experiences, that required a bit of grading, so I’m curious if there are cinegammas or hypergammas that match SLOG-3.cine

    1. FS7 and F5/F55 are 10 bit if recording XAVC in either mode. The colorspace is completely different in Custom and CineEI. In custom it’s basically 709 colorspace, in Cine EI you can choose between S-Gamut, S-Gamut3 and S-Gamut3.cine.

  5. Had this conversation quite a few times now and I had already decided that Slog3 is not for low light. This article explains it very well.

    Like Flemming’s comment above, I get nervous about matching shots, if using Slog3 EI and then switching to custom. If you had a project you had shot 90% in Slog3 S-Gamut.cine and you had to get some low light stuff, would you still go into custom?

    1. Most likely I would go to custom. I’d rather have a small color missmatch than a noisy, grainy, hard to grade image. The average viewer is going to notice the noise and grain more than a small color shift. Generally in low light there isn’t going to be a lot of color anyway.

  6. I thought that S-Log3 was developed to increase the number of steps available in the shadows.
    The way it was explained in an article I read some time ago, full white uses close to all the steps. One stop down, you halve the steps available. 2 stops down, you halve it again, leaving 1/4 the steps. S-Log redistributes the steps so that each stop gets a fairly equal allocation, so it was explained in the article.
    There’s probably some of that redistribution going on with normal gamma curves of REC709, but much more modestly.
    What do you think about this?

    1. S-Log3 only uses 3.5IRE to 92IRE, so compared to most modern gammas that use 0 to 109IRE you already have a restricted data range, then you’re cramming 14 stops into that small range compared to 6 stops in an expanded range with 709. Compared to S-Log2, S-log 3 has a little more data in the shadows, but it’s still a fraction of what a 6 stop gamma like 709 would have.

      Normal gamma curves like 709 use an increasing amount of data for each brighter stop of exposure. This is a good match for the real world where each successive stop has twice the brightness range of the previous one. The first stop in 709 uses about the same amount of data as the first stop in log. But while 709 is going to quickly increase the amount of data in each stop, the increase is much slower with log and once you get to about the 5th stop the amount of data stops increasing.

  7. Hi Alister, it’s your friendly proof reader again! Second paragraph … 970 useable data bits … must have been too late at night, or was it that Raffles beer? It should have read 970 useable levels…. the mistake then runs on into the following paragraphs with things like 70 bits per stop. Not sure what 2 to the power of 70 is, but it is a big number! Once again, thanks for all the work, it is very, very useful, and to quote my late Dad, the man who never made a mistake never made anything. Bob.

    1. That para is correct and that’s the point of log. The total available recording range is 970 data bits (bit’s 0 to 970 for S-Log3). Log uses more or less the same amount of data for each stop whereas conventional gamma’s use almost twice as much data per additional stop.

  8. Hi Alister, I think we are talking about different things. To me, and anyone else with a computing background the term ‘data bit’ means binary data. A ‘bit’ is by definition one binary digit, either a zero or a one. A word length of 10 binary bits gives a range from 0 to 1023 in decimal, that is, 1024 different code values, of which 970 are used. I found your nomenclature somewhat confusing, although obviously others have not. I think my original use of the term ‘levels’ did not help me get my point across. My fault. Not sure if I have explained myself any better this time, but I hope you can see the point I am trying to get make. Bob.

  9. OK, yes point taken. Levels or shades would be more accurate. Problem is than many people with no concept of digital technology have no clue as to what a code value is and when you start to talk about having 970 levels then people question how can you have 970 levels in something that only goes from 0 to 109IRE.

    But yes code values is less confusing and the correct term.

  10. Hi Alistair,

    You confirm my experience concerning Slog3 in low light. But in practice, when you are in documentary shooting, with real life and very very little time to adjust camera parameters (I mean going into the menus) how do you switch from an outside shot in Slog3 to an inside scene in Custom mode? The FS7 takes a while just to switch from one mode to the other.

  11. Hi Alister, the first time I came across the term CV I thought it stood for ‘Component Video’ and rather foolishly decided that the value given afterwards must be the signal voltage in milli volts! It took me some time to realise it was a Code Value. You walk a very tricky and technical path with great aplomb. Bob.

  12. Well I don’t know for the Sony cameras. But for the Alexa what you say is false I think.
    – The 709 image is created from and already Log encoding, like an “s-curve” LUT.
    – Low light is often very high contrast due to light sources being in the frame.
    – If you have details of your scene low, close to noise level, you’ll have your “only” 70 shades per stop (although log also have slope) in Log, but even less in Rec 709 or any good looking curves, cause, if not completely crushed, you’ll be in the slope of the curve, where some stops only have 10 values or less.
    So to be short burning in a lut with crush details that you could otherwise record with a Log format.

    1. What you are saying would only be correct with a LUT applied externally to the camera. The internal processing in a video camera is done at a much greater bit depth than 10 bit. Don’t forget that the Alexa is capable of 12 bit log prores, so at the very minimum the log data must be 12 bit before encoding, so there is more data there. The Sony F5/F55/FS7 cameras process the LUT’d images directly from the sensor data. The LUT’s are not added on top of the log, the images are processed as log+LUT in one single mathematical process so there is no artificial 10 bit log data restriction. Alexa is almost certainly the same as don’t forget you can also have raw + LUT. Internally the image processing is at least 16 bit (the minimum needed for 14 stops of DR) so there will be a lot more data in the shadows available prior to the signal being passed to the codec. Baking in a LUT internally is exactly the same processing process as using any other alternative gamma curve.

  13. hey I love your videos and tips! I have now tested your different HG settings (dowloaded from web) and tested Doug Jensen’s Universal 1 setting, which is based on S-log3 plus some other adjustments he has made in Custom mode,( you have to buy access on vimeo in order to download the image profile). This Universal – 1 becomes the brightest setting I have managed to find. I have also tried out all of the STD with the different options available , all HG versions and S- log2/3 . But none will be as bright as Doug Jensen’s Universal 1 which is based on S- log3. It is perhaps not as much information in the material, but I do not understand witch other picture profile setting that allows the camera to be brighter.

    So Alister please explain if there is something I have not understood…..and pleas share the exact picture profile you use when its very dark outside.

    1. It’s brighter because it’s adding gain.

      Gamma = gain. Gamma is a gain curve.

      Nothing in the camera can make it more sensitive, only changing the sensor will make a camera more sensitive, so anything else is just a gain adjustment whether it’s gamma or gain. So, making the picture brighter through gamma adds noise just the same as adding gain.

      Raising the gamma gain will create a low contrast image with data spread thinly. This will often lead to banding and other artefacts in the image.

      S-Log3 cannot record beyond 92%, so any profile or settings based on S-Log3 will have a constricted recording range. For low light if I want low noise and a good picture I use a fast lens to get as much light as possible on to the sensor and if that fails add light. Anything else will add grain and noise.

  14. Hi Allister
    Thank you so much for your great articles, they help to understand the sony fs7 so much better!
    I’ve done a series of testshootings where you can see that low light with SLOG3 (SLOG2 is a little better) is producing tons of noise.

    However in this testshootings you can see even noise on a late afternoon scene
    > Noise in SLOG3 (fence area), cineEI: https://youtu.be/RGBk7DPN3g8?t=364
    The aperture was wide open and i expected to have nice shadows although in the abandoned rabbit house one can see a lot of colored noise!
    So is SLOG even a problem on cloudy days with reduced light also? The thing is if you are shooting in 4K an want to enlarge a certain portion of the screen it’s getting really noisy in this cases right?
    Thanks Andy

    1. S-Log3 is not producing more noise, the signal to noise ratio of both S-Log2 and S-Log3 is the same. This is a very common missconception based on past experience with standard gammas where what you record is what the end viewer see. With log you have to stop thinking this way and understand that the footage is designed to be graded. After grading if the brightness levels are the same the noise will also be the same.

      See: http://www.xdcam-user.com/2016/07/understanding-the-all-important-signal-to-noise-ratio/



      Log is designed for one thing, a increased dynamic range in a limited recording space. You can’t fit more into a traditional recording space without other compromises.

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