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Grading middle grey when different cameras record middle grey differently

middle grey color chart grading 18% c-log

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#1 Brett Allbritton

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Posted 14 August 2017 - 09:23 AM

I recently purchased a color chart to use while grading and am curious about how to treat middle grey. Intuition would tell me to place it around 50% on the waveform, and articles I've seen on the subject say 45%.

However, I know that different cameras record middle grey differently, for example, C-LOG places middle grey at 32% IRE. I haven't been able to really test it with the C100 I use at work yet, but I'm curious, when grading a C-LOG image, should I correct middle grey to 45-50% IRE, or should I leave it at 32% IRE, and just adjust my highlights and shadows accordingly?

Thanks.


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#2 AJ Young

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Posted 16 August 2017 - 12:32 AM

Yes, you should adjust your gamma curve to place middle grey at 45-50% IRE, but a grey card won't be enough. You'll have to shift the shadows and highlights as well.

 

Your best bet is to use a color chip chart that has values to represent black, white, middle grey, RGB, etc. This helps you precisely adjust your shadows/midtones/highlights as well as saturation, color balance because the chart is designed to align with the waveform and vectorscope.

 

The DataColor Spydercheckr is pretty excellent, and DaVinci can recognize it.


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#3 Robin R Probyn

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Posted 16 August 2017 - 03:23 AM

Its not how a certain camera records a different grey level..   its the gamma curve you are using to record your image .. that changes the grey level.. thats why the C log is 32%.. like all log curves, its low to allow data for the high lights cram in a high DR into a limited bucket.. white level will be even lower that "normal"..like 70% approx.. instead of 90%.. I would have look into the machinations of grading Log footage .. not just only where to put your grey level ..


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#4 Dmitry Romanov

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Posted 21 August 2017 - 12:37 AM

You can solve your task wit 3D Lut Creator easily . Oleg Sharonow have video tutorial about it  . And you can check software on the official website https://3dlutcreator.com/


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#5 aapo lettinen

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Posted 21 August 2017 - 06:48 AM

did I understand the question correctly, you are asking how to grade log raw material fully manually without applying the correct LUT at all? 

 

it would be 100x easier to first add the correct LUT in grading and then continue from there. if your editing software does not allow importing LUTs you can find out a plugin workaround for this or use another software (much easier than trying to manually create a matching s-curve and color settings from the start)


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#6 aapo lettinen

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Posted 21 August 2017 - 07:11 AM

part of the problem is of course that there is no absolute "middle gray" with video cameras because there is gain and signal processing involved and because there isn't only one way to expose a image there can't be one correct middle grey level for all shooting situations (of course there can be but then all material ever shot would look like same-lighting-setup soap opera material without any artistic variations) . shooting a grey card and expecting it to always automatically apply on certain IRE on monitors would basically mean that there would not be any kind of artistic lighting involved, only some kind of idealized tv studio setup...

 

the easiest solution would be to shoot log material with for example camera's standard rec709 LUT applied to the waveforms but recording in the log (for example log-c) , exposing according to the waveform and on-set monitors (if calibrated correctly) , then applying the exact same LUT in post production so that the "middle gray" would be exactly where you decided to expose it on-set using the LUT-applied waveforms and monitors and then fine tune the look after that. Your exposure decisions may call, for example, the middle grey to be 1 stop under with your lighting setup and intended look. 

 

in the log recording the IRE levels can be anything and do not matter much as long as they are recording technically correctly, ie important parts of the image not clipping or having too much compression. if you are only staring at the LOG waveforms and expose every single shot separately with the log waveforms you will have multiple times more work in grading to match the shots because every shot is "graded in-camera separately for technically perfect levels" and thus they don't match in visual look at all 


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