Canon C-LOG exposed, literally

CanonEOS

There are many views on how to properly expose images on digital video cameras from use of waveform monitors, histograms and zebras, but anyone who’s familiar with my approach to lighting for digital acquisition knows I like exposing digital similar to how I would expose film in the sense of not being able to see the image until it was later developed. ie. I use a light meter to expose 18% middle grey at 50%IRE 99.99% of the time. I might shift my middle grey value however depending on the specific scene or if I’m using any special gamma curves to boost specific tonal information in the scene. (On the F3 cinegamma 4 is my all time favourite for boosting the mid-tones)

Having spent the last year to improve my exposure techniques shooting on the digital format using cameras like RED (with a linear and raw based format) or the Sony F3 (with standard or S-LOG gammas) I thought it might be good to delve into the Canon C-LOG to see if I could help clear up any confusion or misnomers on how to properly expose for it.

Whether it’s S-LOG, C-LOG, LOG-C or any other flavour of the LOG gamma curve they all have one common goal and that is to preserve as much data information over the entire dynamic range of the camera sensor as possible. Shooting linear gamma (as RED does) or LOG gamma (as does ARRI, Sony F3 and Canon C100/300/500) are at quite opposite ends of the spectrum in terms of what is happening to the spread of data information used to represent the signal and to some degree the ”lattitude” that can be gained depending on how you decide to expose either format. Refer to my article “When Raw is not Raw” to help understand the key differences between linear gamma and log/curve gamma formats.

When you start working with C-LOG and learning how to expose it properly what’s critical to know is how reflectance values have been re-mapped in the camera giving you “new” exposure levels. Because a LOG image will look extremely flat and lack any contrast it can be very difficult to expose it properly by using your eye, histogram or even a waveform monitor. Your best option is to use a grey card, white piece of paper, and a LCD monitor that can show you the IRE value in a digital readout display. You can also use a lightmeter but since most people don’t use one properly I strongly advise you perform an excercise to determing the effective ASA of your camera and lenses and get familiar with the concept (also your meter might be wrong so this is a worthwhile exercise). You can read an entry I wrote on “how to use a lightmeter“ with digital which should help cover the basics.

The chart below that I put together is based on information provided from Canon’s whitepaper on C-LOG. From the chart you can see how image brightness values are remapped to new values. For example 18% middle grey which is normally 50%IRE moves down to around 32-33%. 90%IRE (white) will appear on your waveform monitor at only 62-63%. It’s really important to use these new values (even if you don’t understand them) when setting your exposure in C-LOG mode since it will maximize the dynamic range your sensor is capable of and preserve as much of the scene information as possible. This “maximum” amount of information is needed later when you de-LOG your footage and begin grading and color correcting in post.

Also observe how changing your ISO on the Canon will impact your exposure lattitude below and above your middle grey point. In essence this allows you to chose an ISO that will let you retain specific image detail for a particular scene. Perhaps if it is low-key, flat, or high contrast. For any DP this an extremely powerful feature because it allows you to select exactly what you want to protect for without having to cheat and shoot using different middle grey values and then push/pull it in post to bring it back to normal.

Remember that anytime you push or pull your image in post you are altering its quality by introducing the possibility of posterization depending on how far you re-adjust exposure, color correct or grade images. Pushing/pulling in post effectively increases (or descreases) the signal-to-noise ratio so the more accurate and consistent you expose your images in-camera when shooting C-LOG it will pay dividends in post and this is even more true if you are using a low bit-depth to record your images.

  1. Thanks for sharing your experience and knowledge
    I have a question
    I m shooting with the C100 and was only able to do a brief test
    When shooting predominantly african american talent, i m thinking about placing IRE around 40 to gain more density on the image
    Have you have experience with similar situations?
    Thanks again
    Francisco

    • Dennis HingsbergDennis Hingsberg05-28-2014

      Your welcome and thank you for stopping by my website.

      In your shooting situation I would typically aim to place darker skin talent 1 to 2 stops over middle grey.

      So 40IRE is a great starting point and would probably serve you well.

  2. Zack BennettZack Bennett10-01-2014

    I’m very confused about the description regarding what happens if we lower the ISO. Wouldn’t it be the opposite? Wouldn’t you gain more highlight detail and lose shadow detail? What am I missing? Great posts by the way.

  3. Dennis HingsbergDennis Hingsberg10-14-2014

    Hi Zack, I know it can be very confusing and I will try to update this post so it is a little more clear. In the meantime checkout pages 4-7 in Canon’s white paper on CLog for more detailed info: http://learn.usa.canon.com/app/pdfs/white_papers/White_Paper_Clog_optoelectronic.pdf

  4. Thank you so much for sharing this! I’ve been working with a c100 for only a few weeks and I was still having trouble understanding certain reactions of the C-Log. It seems a lot clearer now.

  5. Pat HorridgePat Horridge04-30-2015

    A nice article thanks.

    But I do struggle with how people are using this (We run a post house)

    Most Log-C footage I see has little if any information in the upper ranges indicating the shot scene didn’t have the dynamic range of light the Log-C process helps preserve.
    The implication then is that the desired more limited range of captured light has now been captured via Log-C in a smaller 8 bit quantised range (little data if any above 75% or so)
    Isn’t this just effectively making the mid tones more quantised?

    • Dennis HingsbergDennis Hingsberg04-30-2015

      Great post Pat.

      This is definitely a trait of Log based gamma curves where its designed to try and equalize the amount of data quantization across the TOTAL dynamic range of the camera. This presents two issues as you have very keenly pointed out, 1.) if you are not shooting a wide dynamic range scene then you are not using all the data bits available 2.) if you shoot a lower contrast scene (natural or through controlled lighting) then where you place your image relative to mid-point grey means you can end up with LESS data per stop – also not good.

      Maybe a third problem all together is using 8-bits to represent Log, there’s just not a lot of data there to cover off all scenarios:

  6. DanDan08-26-2015

    Helpful post. The white paper is rather long and this sums up what was important to me anyway.

    What I don’t get is why the C100 wouldn’t have Zebras that could go down to 30% IRE. It seems like it would be such a simple feature to add and would allow for run and gun use of C-Log. People don’t really have time to meter ever shot. Especially when shooting documentary. A lot of times I just keep zebras at 70% and try to get my highlights there.

    • Dennis HingsbergDennis Hingsberg08-26-2015

      Glad you found this post useful Dan. About zebras going lower, even Sony did this on their newer 35mm based cameras and the only thing which could explain it is manufacturers like Canon and Sony are expecting people to use zebras for indicating white levels or highlights – not necessarily skin tones. I do know lots of people who use zebras on skin tones but I’m not one of them. I think “white” is far less subjective than all the different colors and tones of skin that might help “good” exposure. Cheers!

  7. KevinKevin05-16-2016

    This is helpful, even though I don’t understand 100% of it all.

    I recently just started shooting with the Canon C100 and have been trying to use the wave form monitor. However, it seems the “monitor” is not giving accurate readings of scenes and is generally saying it is more exposed than it really is. When I take footage that the C100 WFM is saying is well above 100 IRE, in my video editing software, it shows it is well below 100 IRE. Is it possible for the WFM to be out of calibration somehow? When I shoot just using a light meter, and adjust the F stop accordingly, everything looks fine.

    Also, I’ve noticed that C-Log doesn’t seem to tolerate much correction at all before noise is introduced, so it seems to me to be essentially non-gradable. I have noticed when I shoot video to the ATOMOS, I can grade that footage, but with the Canon codec, for whatever reason, it doesn’t seem to tolerate much, if any, grading well. Your article seems to indicate that as well.

  8. AttilaAttila11-15-2016

    I have a question as I am new to this C-log world with my C100mkii. When I want to shoot in C-LOG (AVCHD for now, even if it is not the best I know) and create an infinite white background, what IRE should I see on the WFM when the camera is pointed on the pure white background and when the subject is there as well (note: View Assist is on)?

    In addition I also have a proper white and grey card. I usually use the grey card only for getting the right white balance. For me it gives better skin tones (if that is the right word).

    • Dennis HingsbergDennis Hingsberg01-10-2017

      Typically to get a pure white background you over-expose it by a stop. So if you set skin at 45% IRE (in standard rec709) and you’re white is normally 90%, you want to get your white close to 100% (set to taste) while keeping your subject at the same exposure level as before. It’s very easy to do, but you need to light your subject and background with separate lights obviously to achieve this. Cheers, thanks for checking out the blog.

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