Some friends and me tried to expose log with an 18% gray card.
We used a canon c300 mk2 with the c.log 3 cinema gamut picture profile.
Canon gives the following IRE values for c.log3: 18% Gray → 34,3% | 90% White → 56.4%
But before we tried to use canons values we tried to replicate this workflow to derive the values:
So we set our camera and seconic light meter to 800 ISO and used the spot meter right next to the camera on the 18% Gray chart. We got a value of F 5.68.
Our next step was to set the lens to a F-stop of nearly F 5.68. The waveform gave us a value of 26%.
This is not only a big difference to 34,3%, the value of canon, the picture was clearly underexposed.
Of course, we asked us what our mistake was.
Was it the fault of the video? Is our gray card not standardized? Is our light meter broken? Is a F 5.68 not a F 5.68 on the lens we used, maybe its broken? Is the camera screwing us?
Then we stumbled over this article: http://bythom.com/graycards.htm
The author, Thom Hogan, tells us that cameras don't see 18% gray. Instead they see 12% gray.
This article confused us a lot. We don't know what to think about light metering. What is true and what is a myth?
What do you think about all this? Are you noticing a mistake we made? How is your workflow for exposing log gamma curves? Do you use a gray card, a light meter, ETTR (expose to the right)? Do you use a Bt.709 LUT on your monitor/viewfinder and expose just with that?
And do you understand the 12% gray article and do you think it's true/false?
Thank you for your help!