You probably know that white balancing is extremely important for reproducing accurate colors in your photographs. The question is, when exactly is the best time to set your white balance. Most every digital camera sold has some ability to change the white balance depending on the type of light that you are shooting in (daylight, cloudy, shady, flash, fluorescent, and tungston).
Most cameras also have a setting whereby the camera will pick what it believes is the most appropriate color balance for the light, also referred to as Auto White Balance. The big problem with built-in white balance settings is that they are rigid and don’t take into account the subtle changes in color temperature that can affect the colorcast in an image. Things like atmospheric conditions (clouds, smog) and reflective and translucent surfaces all have an influence on the color temperature. Then add a little mixed lighting like fluorescent lights and daylight from a window and you have a color temperature that no pre-set balance will be able to handle. Even the auto setting will have trouble with scenes like this. Auto settings can also be influenced by colors of objects in the image so once again, not the best choice. This is why many advanced dslr cameras now have custom white balance settings to assist you in getting the best possible color rendition in your images.
Some cameras make setting the custom white balance a snap. All you have to do is set it up in the menu and then point your camera at a white surface and take a picture. All that is left is to select the custom white balance setting and start shooting. Other similar methods use a disc that is placed over the lens when setting up a custom balance. The disc takes all the light in the scene and averages it into its basic color component and records a proper setting for shooting in that particular lighting environment.
These are generally good methods of setting a proper white balance but they are ones that I reserve for shooting in JPEG or TIFF modes. The reason for this is that the white balance that you select at time of shooting is baked into the file and, while it can be changed later in post-processing, it just seems to be best to get it right during capture when all of the other image processing is taking place.
If you shoot RAW, I would advise a different method of setting a white balance. My preferred method is to shoot with the camera set to Auto-WB, shoot a white balance card, and then correct the color in post processing. I use a Whi-Bal card because it is small and has a nice quick release lanyard so it’s always handy.
The card is placed in the scene, ensuring that it is being lit by the same light source as my subject. Then I take a quick picture and then get on with my shooting. If I change to a different location, such as a shady spot, I grab another quick image of the card so that I can keep control over any colorcasts. Once I bring the image into Lightroom or Camera Raw, I use the White Balance Selector tool to click on the card and BAM, instant white balance correction. The other thing I like about my little Whi-Bal is that it has two different greycards, one that is neutral and one that renders a slightly warmer balance. You can also buy white balance cards that are specifically made for portraiture. Once the white balance is corrected, the setting can be applied to all the other images in the series with equally accurate results.
The one thing that you want to do no matter which method you choose is to constantly update your WB as you change settings, lighting setups, or time of day. A passing cloud can make a huge shift in your white balance, which you will have to fight with later if you don’t deal with it at time of capture. To find a whole selection of custom white balance and white card solutions, check out this list at B&H Photo.