Although Auto White Balance will do an acceptable job in many circumstances, it is possible that dominant colors in the frame can skew it to produce an off-color image. It’s the most important argument for using Custom White Balance whenever possible. In the first image Auto White Balance saw the red background and thought it needed to neutralize it by adding cyan to the mix. The model’s skin tones were not enough to average the tones.

After CWB, the camera no longer cares what the colors are because it’s read and recorded the actual color of the light and will not make any shot-to-shot adjustments.

One of the most valuable of all the Menu items, using Custom White Balance (aka Custom WB or CWB) will permit you to neutralize whatever light you’re working under, changing the colors that make up that light, whatever they may have been, to clean, non-biased colors.

Light is not a constant color, except to our eyes and brain. Humans have the cerebral ability to white balance on the fly, interpreting whatever color we see to be neutral, based on our ability to see something we know to be a certain color (white, for instance) and balancing all other colors against that reference. The Custom White Balance function of the camera does exactly the same thing, except that the camera can leave us with a physical record, a print, of a situation that has been neutrally balanced.

To do this effectively you will need to meter and photograph a neutral gray or white card under the light conditions you’re working with. If you’re working under studio strobes, you will also need to use a calibrated flash meter to accurately measure the strength of the light. Either way, it’s important to purchase a neutral target (see How to Calibrate Your Meter for information). Typewriter paper, tablecloths, bridal gowns, or other objects contain chemicals or bluing agents that make them look neutral, but they will not white balance properly because of that chemistry. A commercially available 18% gray target will produce the most accurate white balance.

Every manufacturer has a slightly different way to CWB, and you’ll need to review your camera’s instruction manual to learn the correct way to do it. Any of my Canon books or field guides will show Canon shooters how, step-by-step, with pictures and clear instructional text.

Also check your instruction manual to know how much of the frame you will have to fill to get a correct measurement. I think you should always err on the side of caution and fill the entire frame.

It won’t matter if the picture you make of the gray or white card has an off-color cast. Telling the camera that you want it to neutralize that tone will effectively change the color balance of any additional images. You can prove it by shooting another image of the target, but with the data from the first image used for Custom White Balance. The new shot will show a color neutral target.

Telling the camera that you want to use this balance is effectively telling the camera to neutralize the color it saw in the first image.