White balance (WB) is the process of removing unrealistic color casts, so that objects which appear white in person are rendered white in your photo. Proper camera white balance has to take into account the "color temperature" of a light source, which refers to the relative warmth or coolness of white light. Our eyes are very good at judging what is white under different light sources, however digital cameras often have great difficulty with auto white balance (AWB). An incorrect WB can create unsightly blue, orange, or even green color casts, which are unrealistic and particularly damaging to portraits. Performing WB in traditional film photography requires attaching a different cast-removing filter for each lighting condition, whereas with digital this is no longer required. Understanding digital white balance can help you avoid color casts created by your camera's AWB, thereby improving your photos under a wider range of lighting conditions.
Normally our eyes compensate for lighting conditions with different color temperatures. A digital camera needs to find a reference point which represents white. It will then calculate all the other colors based on this white point. For instance, if a halogen light illuminates a white wall, the wall will have a yellow cast, while in fact it should be white. So if the camera knows the wall is supposed to be white, it will then compensate all the other colors in the scene accordingly.
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