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 White Balance sliders of RAW converters have the greatest impact on color appearance. The numbers used in the Temp slider refer to the Kelvin temperature scale. Even though the opposite is physically true, photographers often describe higher color temperatures as being cooler or more blue and lower color temperatures as being warmer or more yellow. You might think that dragging the slider to the right (higher temperatures) would cool the image, while dragging the slider to the left (lower temperatures) would warm the image. The opposite is true. This is because the slider doesn't create a color temperature; instead, it assigns one. For example, if an image is exposed under 5000K, but assigned a higher value, the image appears warmer. It compensates for the color temperature of light.
The Tint slider is used for fine-tuning white balance, making adjustments between the magenta-green axis rather than the yellow-blue axis. Tint adjustments are generally comparatively minor, with the exception of images exposed under light sources with uneven spectral distributions, such as fluorescent lighting, where more substantial adjustment is often required.
The likelihood that what appear to be neutral values in a scene are truly neutral is low. The best way to ensure that a sampled area is neutral is to include a target that's known to be truly neutral.
Targets need not be included in every exposure. Targets can be photographed in separate exposures. The settings generated by those exposures then can be applied to subsequent exposures made under the same lighting conditions.
Using a target provides an objective reference point, eliminates subjective errors, increases consistency (between sessions and when using multiple cameras) and can speed workflow by saving time and increasing accuracy when batch-processing image files.
Make an exposure of a target every time lighting conditions change significantly, and you'll be able to specify white balance precisely. Make an exposure for each camera used, even individual cameras of the same model and manufacturer, to compensate for unit-to-unit variations. Ultimately, if you use a target, you can choose to use it or not. If you don't use a target, you won't have the option. Keep your options open. Develop the habit of shooting a target.
Besides helping determine white balance, targets can have additional functions. The GretagMacbeth ColorChecker can be used to create custom calibration settings to fine-tune a color palette, either to make it more perceptually accurate or subjectively pleasing. ImagingExpo's ExpoDisc can be used to map sensor dust, to pinpoint vignetting and to identify color shifts across a sensor. With either target, you can create custom compensation settings with your RAW converter or Adobe Photoshop and make routine adjustments automatically.
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