The human eye has a lot of amazing capabilities. It can see in 3D, it has continuous auto-focus, and it is so light-sensitive that it can detect a single photon of light. Amongst these and other astounding feats is the ability to perceive color correctly under any lighting conditions. If this seems like a ho-hum achievement, consider that we have yet to develop an imaging technology that can do this. Film can't, and neither can digital sensors.
What is white balance?
The problem is that different types of light shine with different colors. Sunlight, for example, shines a very blue light (no, our sun does not cast yellow light; take a look at your shadow next time you're outside and you'll find that it's slightly blue), and incandescent light bulbs shine very red. While your eye can adjust automatically to these different light sources, so that color appears correct under each of them, your digital camera must be calibrated to a light source to properly represent color. This calibration process is called white balancing.
If you think back to elementary school, there was probably a day that you learned about rainbows and prisms, and how both of these split light into its component colors of red, orange, yellow, green, blue, indigo, and violet. In other words: white light contains every other color. The idea with white balancing is that if you can get your camera to properly represent white, then all other colors will be correct.
Choose a white balance preset
By default, your camera is configured to use its auto white balance mode. When in this mode, the camera analyzes the scene and tries to determine the best white balance—the one that will yield the most accurate colors. These days, the auto white balance mechanisms on most cameras are very good, and will almost always yield accurate results when shooting in bright daylight, and several other types of light. However, even the best auto white balance system can begin to fail when shooting in shade.
I shot an image in the shade using the auto white balance setting on a Canon EOS 5D Mark II. While the image doesn't look terrible, it is a little "cool"—the subject's flesh isn't quite as warm and healthy-looking as it should be. There are multiple options for correcting this problem.
Most cameras include white balance presets for different lighting situations—usually daylight, shade, cloudy, tungsten, flash, and fluorescent (of which there are often two types). I switched the camera to its Cloudy white balance preset and tried again. Some cameras have a dedicated Shade preset, but mine doesn't. However, a cloudy day is very similar to shade, so it was a good choice.
Set your white balance manually
Shade is not the only type of light that can cause problems. Mixed lighting—sunlight streaming through a window into a fluorescent-lit room—can also play havoc with the white balance in your camera, yielding images with bad color casts. Or, perhaps you're shooting in tungsten light, but standing next to a bright yellow wall. Even though your camera's tungsten white balance might normally yield very good results, the yellow wall can cast the light into a color range that your camera's preset doesn't work for.
For this reason SLRs, and many point-and-shoots, include the ability to define white balance manually. The process different from camera to camera, but the general procedure is the same: Place something light gray (or, in a pinch, white) within the light of your scene and fill as much of the frame with that object as you can. This is your white balance target.
Now activate the camera's manual white balance process. On some cameras you'll need to take a picture of the target, while in others you'll simply frame the shot and hold it while the camera analyzes the target. (If your camera requires you to take a picture of the white balance target, then you'll have an additional step wherein you tell the camera's manual white balance feature to use that specific image.)
I used manual white balance to retake the same image, and as you can see in the photo on the right, it yielded better results than the camera's Cloudy white balance preset.
If you choose to set your camera's white balance manually, you can also use a white balance card, such as the Raw Workflow WhiBal G6 pocket card. The small, $21 card is specifically designed to be a white balance target. It's lightweight, and spectrally neutral, meaning it doesn't have a color cast of any kind. What's more, it's gray all the way through, so if it gets scuffed, you can just sand it off to return to a gray surface.
Remember that a white balance target needs to go within the light that's striking your scene. If you simply hold it in front of your camera, you may not get a good white balance, because you might be standing in different light from your subject.
Don't count on post production corrections
Because digital image editing software is so powerful, a lot of people think "I don't have to worry about white balance, I'll just fix my image in post production" but this is the wrong approach to take for white balance if you're shooting JPEG images. That's because it's often extremely difficult to fix a JPEG's bad white balance with a photo editor, and sometimes outright impossible. For those times when you can fix it, you may find that performing any additional edits leads to bad artifacts in your image, such as shadow areas that end up looking chunky and banded. Therefore, it's best to get white balance right in-camera when shooting JPEGs, both to save yourself editing hassle later, and to ensure good results. Alternatively, you can choose to shoot raw.
The Raw solution
If you want to avoid white balance troubles altogether, just shoot in raw mode. One of the great advantages of raw files is that you can alter the white balance in a raw file after you shoot, and achieve the exact same results as careful, manual white balance.
For times when you forget to white balance, or when it's impossible to achieve correct white balance (low-light shooting, shooting a distant object in a different type of light from where you're standing, or shooting stage productions lit with multiple-colored lights can all be impossible manual white balance situations) the ability to alter a raw file's white balance can mean the difference between getting and missing the shot.
Film photographers have to be certain that they use film that is balanced for the type of light in which they are shooting, and if that isn't possible, then they have to work hard to add gels to lights and windows to correct the light for their film. Digital photographers have it much easier, but only if they take the time to learn and use their camera's white balance settings.
[Macworld senior contributor Ben Long is the author of Complete Digital Photography, fifth edition (Charles River Media, 2009).]