Wednesday, June 27, 2007
Double-Processing White Balance
Whether your goal is perfect neutrality or creating an effect, you can use this technique to refine the colors in your images
The settings cameras provide are useful generalities. They're not specific to individual lighting situations and don't incorporate unit-to-unit variations. Consider camera white balance settings useful starting points. Some use these settings to deliberately alter white balance for creative effect, producing an image that's warmer or cooler than a given scene. Optional postprocessing with a RAW converter allows you to further refine white balance.
File type is essential for exerting maximum precision and control over white balance. JPEG files, which are derivatives of RAW files processed in-camera, have fixed white balances. RAW files offer you the flexibility of customizing white balance.
White Balance With RAW Converters
Like digital cameras, RAW converters offer general settings for white balance. Like digital cameras, settings typically include As Shot, Auto, Daylight, Cloudy, Shade, Tungsten, Fluorescent, Flash and Custom. These are model-specific, but not unit-specific generalities. Just starting points, these settings typically require further adjustment when postprocessing files.
You can create new settings for each of these light temperatures that will be specific to your individual camera. Photograph an appropriate target (such as a GretagMacbeth ColorChecker), use the White Balance tool to sample the white point, and save the new custom settings (with an appropriate title, including light temperature and camera model). Again, while more accurate than manufacturer defaults, these too will be generalities.
You also can customize white balance settings on a per-session or individual image basis using a RAW converter's White Balance tool—if an image includes useful target values. When using this tool, take care with originals containing significant noise; the noise can adversely affect accurately assigning white balance.
Choose a large enough area to average the noise in a sampled area. If you zoom in, a smaller area will be sampled; if you zoom out a larger area will be sampled.
Favor light grays; neutral, nonspecular, textural highlights are best choices. Very bright whites are rarely good target values as they often include significant amounts of hue (especially if one or more channels are clipped); when chosen as neutral targets, they will inaccurately assign white balance. (Unlike the White eyedropper or Gray eyedropper in Photoshop, the White Balance tool doesn't affect the luminance of an image.)
Using a synthetic target to assign white balance is the most precise method. Further hue adjustments to a file are typically hue-specific or subjective in nature.
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