Well, I thought I knew this stuff and it was simple but ...
(You can ignore all this preamble unless uyou think it's relevant)
Tomorrow we shoot the first scene of a film that's been in the pipeline for a while. The scene is being shot in a pub in the daytime. Perhaps unimaginitively we have two blokes sitting opposite each other at a table. The shot will comprise a master and opposing OTS.
There is a window a a few paces away begind one of the men. Pubs being what they are nowadays the window has an LCD TV screen immediately in front of it, thus reducing the vast majority of the light coming in although there is clearly some.
Not wishing to leave too much to chance, we ran a technical rehearsal on Monday. Obviously we set up so the Window and its ugly TV screen were not on the frame. We used the wall mounted tungsten lights (again kept out of shot to avoid burnt out bits) a few LED 160s with tungsten filters, and a relector (this seemed to work better than direct light - much as you'd expect).
When I came to examine the footage, the shots of the man facing the window looked pretty good (I'd expected this to be the problematic shot as he was the one lit by more of a mix of tungsten/LED and natural(. The guy shot from the opposing direction though looked decidedly green. I improved it a bit in post and can probably do better, but I'd rather get it right from the off.
OK, I thought, that's because the camera op just used the in-built auto white balance (Camera is a Sony FX1 - the consumer version of the Z1). What we need to do is set the white balance manually each time we reposition the camera.
So, off I trotted to his place to demonstrate. And here's where I now feel I've lost the plot.
We managed a similar but more extreme setup in his house. One end of the room joined on to a conservatory so there was plenty of natural light coming in from there. We lit the room with tungsten.
I stood at the conservatory end of the room (position "A") and performed a WB (in the time honoured tradition of holding a piece of white A4 in front of his face) Lets call that WB setting X. The resultant looked great. Keeping the same WB setting I moved to the other end of the room (position "B") the image was still pretty good (although he was not lit well enough - as I said this was more extreme than in the pub)
I then set the WB again from this new position (Position "B" WB setting "Y") The resultant image was decidely cool in colour and certainly wouldn't match the first image. For example, he was alongside a palish lemon coloured wall. From "position A" using WB X the wall was clearly pale lemon. From position B using WB X the wall was still clearly pale lemon - though much paler as one might expect with external light reflecting off the wal towatrds the camera - ie it was quite natural.
Using WB Y, the lemon wall looked, if anything a very slightly bluey white from position B, and only barely yellow from position A.
So these tests would suggest to me that when I come to shoot tomorrow, once we've set the WB and got an image we like, we should leave it well alone even though we are moving to different positions within that environment. Is this correct?
What I would really like to understand though is why, when setting a white balance from position B above, were the results so dreadful? (I performed the operation several times to double/triple check, and the camera never complained that it was struggling to get a good WB reading.