actually white is gray, digital camera or video camera, after you set the WB, no matter it is pure white, light gray, dark gray or even black, the TTL system will expose everything to 18% gray.
WB don’t copy color
most people special video professionals, they think that when they WB on a white paper, the camera copy this color and apply to their image, so that a white paper under a hot yellow light will change back to white just like the white paper, so they never WB on gray and worry their image will change to gray, this is very wrong, WB don’t work like this.
so how does WB works?
WB work with RGB values, in a digital camera or video camera point of view, white is make up of [R=255 G=255 B=255], gray is [R=128 G=128 B=128] and black is [R=0 G=0 B=0] with this in mind, under a bright sunny day, a gray card may have a values of [R=128 G=128 B=128] all values equal but under any warm yellow Incandescent light it may have a values like [R=128 G=128 B=100] the blue values has decreased, so WB system know blue channel has decreased, and will try to balance it back to 3 equal numbers to color correct the yellow cast.
why WB on white is bad?
because WB need rooms to detech color cast, since a channel only has 255 step of grays, when you expose to either side white or black, the top or bottom information has clipped, and the WB system don’t have enough information to correct the cast, so a gray to you is a color, but in a WB system gray is information how much it can correct the color cast, so this is why gray is always better than white.
I have seem alot of video professionals that works in TV stations and wedding, they WB on a piece of paper, most of the time i take closer look at their screen, yellow cast is still there, and sometime WB on white paper will make everything a very light green, but most people don’ care about it.
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