DV Info Net - Adjusting for White Balance Question

Here is Walter Graff's response to this same question in DVXUser

"Here is a sort of non technical look at white blance. White balance is a relatively simple process. Your camera uses the green color channel as a reference for every thing it does. So red and blue are always compared to green and you get the correct colors of the rainbow. So when you hit the black balance on a camera you'll see that the viewfinder shows black and suddenly the black seems to change levels. What you are seeing is the following. The camera looks at green and then compares the red to green and the blue to green if the red and blue do not rest on the same plane as green then the camera quickly makes them identical. That is what the different shifts of black are in the viewfinder when you hit white balance. Its first the green channel, then the red and blue channels being compared to green and adjusted. White balance is similar but it uses the white peaks of the colors and makes the appropriate adjustments. So the idea is that if you aim the camera at something white it will reflect the color temperature of the light hitting the white surface. The camera sees where green is in that reflection and makes red and blue the same. And when its done you get a camera that sees white the way you do. Actually (you have an auto white balance in your head that makes adjustments on the fly). So the secret to a good white balance is to fill up your frame with at least 80% of something white. It could be a white sneaker (tennis shoe), white piece of paper, a white shirt, etc. And that white need only reflect the color temperature of the room you are in. If its a mixed color temperature you have to decide which color temperature you want to predominantly be represented in a shot. So if your room is all 3000k and you have a little window in the background with 4600k outdoor light, but WB for just the 3000k, you get correct color for the room and the window looks blue. This is ok. It IS blue outside and you will not be able to find a perfect balance at such a color temperature extreme. You don't need to. But say you are in a room with 4000k ceiling florescent's and you are going to supplement your lights with 3000k lights. You could gel lights but why bother. Simply adjust your white card so it get s a bit of both color temperatures and you will get a balance of the two and a good white balance. Use a monitor to see the results. If it looks to blue WB again and get more flouro and less incandescent. Too orange, get more incandescent and less flouro. If you remember from my lecture ( I gave everyone a swatch book), you can use gels to help crate subtle WB effects. Warms cards are a waste of money and you can do the effect better like this. If you have a Lee dealer you know, ask for a cinematographers swatch book. If not check the website. I think you can order one free. This swatch book contains about 50 4"x4" gel samples including color temp blue (CTB) and color temp orange (CTO). WIth these you can do the exact same thing as an $80 warm card. So remember I said that white balance was the comparison of red minus green and blue minus green till they were all in the same place? But what if I took apiece of 1/8th CTO from that swatch book and placed it in front of the camera so that when I aimed my camera at something white, the white had an extra amount of that orange gel? If I white balance the camera will subtract more orange giving me more cyan and that would make my picture cooler looking. And the opposite is true. If I took an 1/8 CTB and did the same, I would add blue giving me more amber and the picture would look warmer. Using 1/4 of either CTB and CTO gives me appropriately more of each color. That is what a warm card is. So don't waste your money on one of these when you can do the same trick to more of a degree with a pice of gel. These premade plastic white cards and warm cards are fine if you want to use them but they get old, fade, get dirty, etc so eventually you might not have such a white card. And white is everywhere so once again its free."

http://www.dvxuser.com/V3/showthread.php?t=20201