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False Color on ARRI ALEXA - LUT or LOG C based?

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    False Color on ARRI ALEXA - LUT or LOG C based?

    For those who know and have used the Alexa's False Color for exposure both LUT and LOG C based - which do you prefer and why when working on a professional set?
    Is there a de facto standard?

    As I understand that on paper it might be better to use FC LOG C based so you see what you are actually exposing to - on the sensor -, most people, including me, tend to use FC based on the LUT used for production (mostly ARRI 709).

    I in fact like that the standard ARRI LUT is on the more contrasty side, because I then tend to overexpose a little, which in the end protects my shadows. And even with that, I know in the back of my head that the LOG leaves even more room to work with in post. It's kind of nice to rely on that.

    BUT: If there are reasons to absolutely use FC LOG C based - I am eager to know!
    Interested in how the majority of you is handling this.

    Thanks a lot, everyone, and best regards.

    #2
    I thought it was always been common practice to use it with Log C, but maybe both are commonly used.

    As you say, I think it's best to see what the sensor is doing, but because the camera has such excellent under/over and you're using the native LUT which might come close to your final product, I don't know if it matters much unless you'd be using a non-traditional LUT and completely seeing something else (or doing something else in post).

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      #3
      Thanks for chiming in, NorBro.
      Much appreciated.

      Exactly what I am thinking... It shouldn't actually matter much because it's an Alexa.

      But: If you use it LOG C based, you have to pull everything down in post SO much that at some point I feared I am actually losing information somewhere.
      That's when the question popped up in my head. And the web is suspiciously quiet when you google the issue...

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        #4
        I think in practice (in my mind), if you're using Log C then you are seeing more latitude in the highlights and exposing more to the right thus you're able to introduce more information in the shadows.

        So then when you pull it all back down, I would think you'd have more information, you know?

        But if you're exposing for Rec709, you're being conservative with the highlights since you're not seeing all of the latitude properly thus the shadows aren't getting as much information.

        It also depends on the scene and what you plan on doing in post. For me, it was always about the white blobs...try to control/limit those overexposed areas as much as possible (the windows, reflections, skies, etc).

        Sometimes with Rec709 they are so strong that you're trying to control them too much and you end up underexposing.

        At some point, it's still only 14 stops and not 20+ like our eyes, so we do the best we can.

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          #5
          That makes a lot of sense to me.

          After a long search (again...) I found this passage that only seems to exist in the pocket guide for the Alexa XT (and sadly not in all following ones, for whatever reason), and that kind of (!) clarifies things:

          "Rec709, P3 and Log C share very similar levels for middle grey, but differ in the bright and dark areas as Log C has some additional range beyond what the false color exposure check indicates. Looking at it from the other side: if the false color exposure check indicates that you are OK in Rec709 or P3, you are also going to be OK in LogC."

          Which basically says: It does not really matter much, which base you choose for FC if you focus on middle grey/skin tones when exposing (which we all do), as those indicators (GREEN and PINK) will not really differ from each other either way.
          If you want to be able to see more of what's going on way up and down, choose LOG C based.
          Right?

          This is how I understand it now.

          And it explains why people might actually have more of a personal preference and take on the matter than being forced to choose by strong technical facts.
          As this is not my mother tongue, would you agree with my explanation?

          Thanks a lot, NorBro!

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            #6
            Yeah, I would agree - although the comment about being OK in LogC is just saying that you'll be OK; not that you might have more room for more exposure (which you might).

            Also, exposure just like grading and saturation and contrast, etc. is subjective as well; some people like brighter, and some like moodier, darker scenes. Or certain exposure for various skin tones.

            ___

            But at the end of the day, the bottom line is it's not going to make a big difference for most people if they understand the camera and how to expose in challenging situations (plus sometimes grading completely changes the look).

            If you're comfortable with Rec709, use that. And maybe sometimes in extremely harsh conditions switch to LogC so you can take a peek at the sensor levels a bit more and see how much flexibility you might have.

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              #7
              BTW...something else you may or may not know:

              Some camera operators monitor LOG on their own monitor/VF while sending out a Rec709 via SDI/HDMI to the video village (clients/etc).

              Just wanted to mention that because in the context above when you mentioned Rec709 is used by most people then that would be the case for practically everyone just watching (but not always the case for the 1-2-3 people closest to the image creation who have either option, sometimes favoring LOG with other exposure tools besides FC like zebras or a waveform).

              Personally, my brain was never wired for false color. It distracted me too much, and I prefer other exposure assisting.

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                #8
                I fully agree.

                That's very good info.

                I know there's feeds going out to the director and other parts of the crew that will be clean besides the Rec709 LUT. I basically meant those people immediately responsible for setting exposure.

                I may just get used to using it LOG C based.
                After this conversation, it holds more pros than cons.

                After all, it is what ARRI recommends in saying it makes the most sense for LOG C based projects. And I'm not going to argue with those guys. :-D

                Good exchange, thank you.

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                  #9
                  Short update: It works like a charm. Not going back to LUT based any time soon.

                  ONE downside, though, and it was to be expected: As you tend to expose more to the right, focus peaking gets... tricky, to say the least.
                  Much less contrast for it to work with.

                  But I know that's only a concern for a single operator show without a tripod, when you have to fully rely on yourself and use the flip out monitor for focus pulling.

                  Not a problem if you feed a secondary monitor and do focus pulling on that one with whatever LUT you prefer.

                  Learned a lot from all this.
                  Last edited by Philipp DW; 08-04-2021, 05:20 AM.

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                    #10
                    Yeah, good point; it's definitely challenging with the LOG image.

                    I haven't manually focused in years but I know what you mean.

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