Every light source emits a particular color cast. The old-fashioned fluorescent lights found in most public restrooms, for example, put out a bluish-greenish light, which is why our reflections in the mirrors in those restrooms always look so sickly. And if you think that your beloved looks especially attractive by candlelight, you aren't imagining things: Candlelight casts a warm, yellow-red glow that is flattering to the skin.
Science-y types measure the color of light, officially known as color temperature, on the Kelvin scale, which is named after its creator. You can see an illustration of the Kelvin scale in Figure 6-16.
When photographers talk about "warm light" and "cool light," though, they aren't referring to the position on the Kelvin scale — or at least not in the way we usually think of temperatures, with a higher number meaning hotter. Instead, the terms describe the visual appearance of the light. Warm light, produced by candles and incandescent lights, falls in the red-yellow spectrum you see at the bottom of the Kelvin scale in Figure 6-16; cool light, in the blue-green spectrum, appears at the top of the scale.
At any rate, most of us don't notice these fluctuating colors of light because our eyes automatically compensate for them. Except in very extreme lighting conditions, a white tablecloth appears white to us no matter whether we view it by candlelight, fluorescent light, or regular houselights.
Similarly, a digital camera compensates for different colors of light through a feature known as white balancing. Simply put, white balancing neutralizes light so that whites are always white, which in turn ensures that other colors are rendered accurately. If the camera senses warm light, it shifts colors slightly to the cool side of the color spectrum; in cool light, the camera shifts colors the opposite direction.
Snow, water, shade
Bright sunshine Fluorescent bulbs
Tungsten lights Incandescent bulbs
Figure 6-16: Each light source emits a specific color.
The good news is that, as with your eyes, your camera's Automatic White Balance setting, which carries the label AWB, tackles this process remarkably well in most situations, which means that you can usually ignore it and concentrate on other aspects of your picture. But in some lighting conditions, the AWB adjustment doesn't quite do the trick, resulting in an unwanted color cast like the one you see in the left image in Figure 6-17.
Serious AWB problems most often occur when your subject is lit by a variety of light sources. For example, I shot the figurine in Figure 6-17 in my home studio, where I use a couple of high-powered photo lights that use tungsten bulbs, which produce light with a color temperature similar to regular household incandescent bulbs. The problem is that the windows in that room also permit some pretty strong daylight to filter through. In Automatic White Balance mode, the camera reacted to that daylight — which has a cool color cast — and applied too much warming, giving my original image a yellow tint. No problem: I just switched the white balance mode from AWB to the Tungsten Light setting. The right image in Figure 6-17 shows the corrected colors.
d\NG/ Unfortunately, you can't make this kind of manual white balance selection if you shoot in the fully automatic exposure modes. So if you spy color problems in your camera monitor, you need to switch to either P, Tv, Av, M, or A-DEP mode. (Chapter 5 details all five exposure modes).
The next section explains precisely how to make a simple white balance correction; following that, you can explore some advanced white balance features.
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