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For many years, we've been told that color casts—those shifts in color towards blue or yellow—are a bad thing and should be corrected at all costs. In the film days we used color-correction (CC) filters to battle them and, in the digital age, most choose to set their cameras to auto-white-balance (AWB), in effect telling the camera to detect and neutralize color casts automatically. After all, neutral whites and lack of color casts are desirable and natural, right? Wrong!
A couple of years ago, a workshop participant brought with them a little device that mounts to the front of the lens and allows for on-location white balance calibration. The device was still new and in its original packaging, which showed "before" and "after" samples. I had to admit that, to my eye, the "before" sample actually looked better than the "after" one. The subtle color cast added a nice mood that was missing from the sterile "corrected" version.
After a short discussion with the class, it became obvious that many did not, in fact, fully understand the concept of white balance and that color casts are not only perfectly natural but sometimes very desirable.
To understand white balance, we must first understand the related concept of color temperature. In very simplistic terms, color temperature is a measurement of the color of light. Natural light is not perfectly white and shifts along a scale ranging roughly from yellow/amber to blue. The same is true for various sources of artificial light. For example, the recent debate around Compact Fluorescent Lightbulbs (CFLs) is largely about the fact that traditional tungsten bulbs emit a much warmer (yellower) light than the cool (blue'ish) one of most fluorescent bulbs. The reason is that tungsten and fluorescent have different color temperatures. Similarly, sunlight will not have the same color temperature early in the morning as it does in the middle of the day; it will also not have the same color temperature out in the open as it will in a shaded area. Color temperature changes all the time, which is perfectly natural.
Color temperature is measured in degrees Kelvin (°K) and, confusingly, the lower temperatures are on the yellow end of the scale, which most people intuitively think of as "warm" colors, and the higher values are on the blue end which, conversely, is often considered "cool". For example, the temperature of candle light is around 2000°K; mid-day sun is around 5500°K; and cloudy/overcast skies are at around 6500°K, etc.
When your camera is set to AWB mode, it will attempt to detect the ambient color temperature and neutralize it so whites always appear pure (i.e. containing equal amounts of red, green and blue). In practice, if your scene is lit by candles, for example, AWB will eliminate their romantic warm glow. If your image was made on a cold and cloudy winter day, AWB will ignore the slight blue hues and, instead, will make the scene appear warmer than it truly was, effectively eliminating its wintry feel. In short: AWB can be a real mood killer.
There are, of course, cases where neutral color is very desirable. For example, in product photography, your client will want their logo and product to appear consistent wherever it is displayed. And, if you are a wedding photographer, heaven help you if the bride's dress is not perfectly white. Still, these are special cases and not the general rule for most photographic situations. In particular, outdoor photographers will do well to pick a consistent setting for their white balance (e.g. "Daylight") rather than let the camera make the decision for each exposure.
Differences in color temperature also make a good argument for working in RAW rather than JPEG. RAW files are tagged with the camera's white balance setting but this setting can be changed after the fact during the conversion process, while working on a color-calibrated display to achieve the optimal setting. When capturing in JPEG, the white balance settings are used by the camera's computer to determine all colors in the resulting file, and while possible to change them to a degree after the fact, will require more work and not be as accurate.
On a recent trip, I set out to photograph desert wildflowers. I scouted the location the day before and decided to return to it before sunrise the next morning. I wanted to capture the quiet mood of the pre-dawn light, which is, naturally, very blue (i.e. has a high color temperature) and the way it complemented the vibrant golden blooms as seen below on the left. For comparison, I also exposed the same scene with the camera set to AWB. The result is a muddy, lackluster sky that looks and feels nothing like the mood I was after (below, right).
If your images feel a bit too warm or too cool and don't always represent the mood you were after, AWB may be the culprit. Capture in RAW and adjust your color temperature post-exposure to match your vision.
You can see more of Guy Tal's work at his website and read more of his writing at his blog. Chris has previously written about neutralizing the color of an image in this blog post if you're interested.