The color of gray that the camera sees is completely dependent on the color of the light source reflecting off of the card.
I seldom have the opportunity to do a manual white balance at stage shows, because when I arrive to set up cameras, the house lights are on and the curtain closed and I have no access to the "actual show lighting" until the show is going. So I just use the "Incandescent" setting on all cameras and that has worked well for me. Lighting can and will change throughout the show also, so at least if all cameras are set the same, the color is consistent and I can make tweaks when editing.
Of course, when I have the opportunity I will always go for the manual white balance (or preset that looks good) for best results.
Different light sources (sunlight, overcast, incandescent, flourescent, etc.) all have different color "temperatures". Some are warmer (redder), some are cooler (more blue), and so forth. I vaguely remember some of this from sixtth-grade science class, with "kelvin" numbers and such. The human eye compensates for this automatically and always see white objects as white, and other colors then follow suit. Video cameras are not so good at this - AWB attempts it, but doesn't always get it right. That is why the presets and manual option, to TELL the camera, THIS IS WHITE. Once it is calibrated to what "white" is, then all the colors look correct under that particular light source.
To a degree, footage shot with the wrong WB can be corrected during editing, but often it can be so far out of whack that it never looks quite right. There is no substitute for getting it right when shooting! If someone looks like a blueberry or a sun-burned Martian on tape, good luck getting a nice skin tone in post....
Safe Harbor Computers
Whatever your white balance is, if you use a follow spot, make sure you color correct the spot light too with gel to match your white balance. Otherwise your colors could be great but your talent might be blue if you WB to 3200.
Also, with a follow spot, once it is color corrected, if you dim the intensity on the spot light, it is going to change the color temp. Your talent may look a little orange.
I have done a lot of TV shows in theaters, moving lights and spot were at 56k and pars and other accent lighting was 32k.
We used to white balnace to a gel color corrected spotlight.
Hello all and thanks for your interest, I set my white balance using warm white led lighting with my grey card. I feel that led warm white comes somewhere between filament lights and really cold lights so is fairly middle of the road. I suppose my real question is once you have set your white balance in this way is it standard practice to try to compensate further or leave things as they actually were ?
For the most accurate colour reproduction, you should manually white balance off a grey / white card which is illuminated by the light source which is illuminating the scene you are recording. This is assuming that there is not a gel in the light to change its colour.
Thank's for all your replies I understand now