Quite simply, the brain adjusts the color that we see. The vast majority of the time, we are not even aware that this is happening. For instance, when you walk from a parking lot into a building, you are probably not even aware that the color of the light has changed significantly. For photographers, this creates a problem: the brain monkeys with the color we see. Thus, we do not always see color correctly. In particular, when dealing with warm or cool colored light, the brain functions on a principle of constancy. It expects the colors of objects to remain fairly constant throughout the day. If the color of objects changes during the day because the light that is illuminating them changes, the brain tends to filter out at least part of that color change. As a consequence, the color that we see during times of strong warm or cool light tends to be less intense than the color of the actual light and, possibly, less intense than what the camera will see and record. For instance, a casual photographer may not be aware that the light has become warm toned in the afternoon until he gets his film back and discovers that his wife's skin has an odd yellow tone.
Before getting into the tools, the concept of color temperature needs to be introduced. Color temperature is just a way of quantifying the color of light. It is measured in degrees Kelvin (K). Normal daylight has a color temperature of around 6,500K. Warmer light has a lower color temperature. The warm light that occurs late in the afternoon might have a color temperature of around 4,000K. Cooler light has a higher color temperature. The bluish light that sometimes occurs in twilight periods of the day might have a color temperature of about 7,5000K. So, our concept of warm and cool light is tied directly to the color temperature. The more warm (yellow) the light is the lower the color temperature; the cooler the light (blue) is the higher the color temperature.
The temperature of the light illuminating an object is extremely important. Experienced photographers strive to match the light to the photographic subject and the mood that they are trying to create. However, just having the right light temperature is not enough. The photographer must capture that color temperature in a way that correctly portrays what the photographer intends.This is demonstrated in Figures 1 and 2. The image in Figure 1 was shot on a beach during the last few minutes of a colorful sunset. The sunset was backlighting the birds and illuminating the sand with a golden hue. In this Figure, the temperature of the light was correctly set. Figure 2 is the exact same image except that the temperature was deliberately set to an improper value. As can be seen, the entire mood of the image has been lost due to the incorrect temperature setting.
Color Temperature and Digital Cameras
Color temperature is also important for those that use digital cameras. Some of you may be thinking, "I have used digital cameras for years and have never had to worry about color temperature". You may not have worried about it, but the camera did. The camera manufacturers knew that the color of the light would affect the colors delivered by the camera. Therefore, they decided to deal with the problem by designing the cameras to automatically measure the light temperature and to make adjustments as the light changes color. That is why you can shoot in the relatively neutral light with your digital camera in the afternoon and then shoot the next day in the cool light of early morning and still, probably, get reasonable color in both situations -- even though the color of the light was different. Your digital camera corrected for the change in light temperature.
The function on the camera that does this is called the white balance function. When the camera does this automatically, it is called auto white balancing (not surprisingly). So, if the camera does this automatically, why even bother discussing it? Well, it turns out that many cameras have other white balance options than auto white balance. While the auto white balance on the newer digital cameras is pretty good, it is not perfect. In some cases, you may not get the color you expected or desired when using auto white balance. When that is the case, one of the other white balance options may be more desirable. More advanced cameras generally have three types of white balancing: 1) automatic, 2) preset, and 3) custom. When all else fails, photographers that use the raw format can set the white balance manually. Since each of these white balance options has advantages and disadvantages, and can change the colors in an image, it is important to understand each option.
Auto White Balance
With auto white balance, the camera attempts to determine the color temperature of the light and automatically adjust for that color temperature. Many people just leave the camera set to auto white balance all the time. This is certainly the easiest option. Auto white balance works reasonably well under the following conditions: (1) the application does not require absolute maximum color accuracy, (2) there is not a preponderance of one color in the scene being photographed, and (3) the photographer wants adjustments made for the color temperature of the light. It is also a good option for situations where the light changes over time and speed is an issue (e.g., animal photography). This is usually the case for general family and vacation type photography. It is often true for even serious landscape and animal photography. After all, who would even know if the blue color of the sky was off by 2%? Figure 3 shows an image that works well with auto white balance. This image is a landscape shot where reasonable, but not fanatical, color accuracy is required. Also, the image has a mixture of colors without one color dominating the image.
Figure 3: Good Image for Auto White Balance
As mentioned in the previous paragraph, in auto white balance mode, the camera does its best to determine the color of the light and make adjustments. However, the methodology that is used to do this requires that certain assumptions be made. These assumptions do not always match perfectly with the scene being photographed. As a consequence, the auto white balance option does not always yield perfect results. Accordingly, photographers may experience problems using auto white balance when the conditions listed above are violated. Therefore, auto white balance may not be a good choice if:
Absolute Color Accuracy is Required: If you were to photograph the introduction of a new fall line up of a major fashion company at a New York fashion show, you would not want to risk using auto white balance. If the color of the models' outfits in your photographs came out slightly off, you would no longer be employed in that industry.
There is a Lot of One Color in the Scene: The preponderance of one color can fool the auto white balance function into assuming that the light has a lot of that color in it. This can result in an incorrect white balance setting and a color cast in the photograph.