ON THE LEVEL The term "white balance" is more literal than you might think. Video, as you know, is composed of red, green and blue (RGB) components, and it’s an additive color system where these components add up to white. When the RGB components of the image have different relative levels, they’re out of "balance" with each other. The result is an image that’s overly red, blue or worse…green. You quickly can visualize this on a waveform monitor that shows the RGB components of video. By adjusting the white balance of the video, the RGB levels are all the same, "balancing" the image. (See the properly balanced image on the following page.)
That’s pretty straightforward, but what about that whole Kelvin temperature thing?
White balance is usually expressed in terms of a Kelvin temperature value. This is because the color temperature of a light source can be expressed as the physical temperature of an ideal black body radiator or an ideal light source that emits all spectrums of light. You’ve probably seen this scale before, where warmer (red) light has a low temperature and cooler (blue) light has a high temperature. On cameras, this scale is used to describe the color-level adjustments done to RGB in order to match the Kelvin temperature of light that you’re shooting. In indoor situations, lit with traditional tungsten light bulbs, the temperature is around 3200 Kelvin, where daylight is somewhere around 5600K. Most cameras have these values as preset white balances, either described in Kelvin value or with those little light-bulb and sun icons. Cameras are calibrated at the factory so that these presets will produce white balance under the given lighting scenario. But that brings up our first question, which is, "Why aren’t my preset white balances producing a proper white balance?"
WHAT’S WRONG WITH MY PRESET WB?
As you may have noticed, most cameras have several preset white balance values for shooting under various lighting conditions. These settings often can be accurate, but there’s only so much a camera manufacturer can do to make sure they hold true. The photosites on your digital sensor are analog light gatherers, which vary based on temperature, age and other factors. Meaning, they don’t always produce the same result over time, which is one factor in presets not actually being accurate. The other very big factor is your lens; different lenses produce different colors, which is sometimes considered a good thing. For instance, some lenses are felt to give a warmer look. However, they do throw the white balance off and make your presets no longer accurate. What can be done to fix this, you ask? Well, there’s a process called "white shading" that does just that.