Reason: All color films are standardized, or balanced, for a particular "color" of light, and digital cameras default to a particular "white balance." Both are measured using a scale called color temperature. Color temperatures were assigned by heating a mythical "black body radiator" and recording the spectrum of light it emitted at a given temperature in degrees Kelvin. So, daylight at noon has a color temperature in the 5500° to 6000° range. Indoor illumination is around 3400°. Hotter temperatures produce bluer images (think blue-white hot) while cooler temperatures produce redder images (think of a dull-red glowing ember). Because of human nature, though, bluer images are called "cool" and redder images are called "warm," even though their color temperatures are actually reversed.
If a photograph is exposed indoors under warm illumination using film or a digital camera sensor balanced for cooler daylight, the image will appear much too reddish. If you were using a slide film, you'd get reddish slides. The photoprocessing lab can add some blue while making prints from "daylight balanced" color negatives exposed under this warm light, though, giving you reasonably well-balanced prints. Some professional films are balanced for interior ("tungsten") illumination. If one of these is exposed under daylight, it will appear too blue. Again, prints made from tungsten-balanced color negatives can be corrected at the lab.
At the same time, your digital camera expects to see illumination of a certain color temperature by default. Under bright lighting conditions it may assume the light source is daylight and balance the picture accordingly. In dimmer light, the camera's electronics may assume that the illumination is tungsten, and color balance for that. This may be what happens when your digital camera's white balance control is set to Auto. Of course, digital cameras, being the sophisticated beasts they are, can often detect white balance fairly accurately with no input by you. Some electronic flash units can even report to the camera the particular white balance which they are outputting, since a flash's color temperature can vary depending on how brief the flash exposure is. Figure 6.13 shows an image exposed under tungsten illumination with a daylight white balance.
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