Monday, June 21, 2010
All About Color Fringing—06/21/10
The causes and fixes for crippling chromatic aberration
Chromatic aberration is especially fixable when working in a RAW workflow. Adobe Camera Raw, Lightroom and virtually all RAW processing engines offer color noise and aberration controls of some sort. Often, zooming in to preview a detailed portion of the frame at 100%, combined with adjusting the pertinent chromatic aberration sliders on the RAW processor, can minimize fringe or eliminate it altogether from your files. Further adjustments can be made with third-party noise-reducing programs such as Noise Ninja, which has a Lens Correction feature designed not only to eliminate vignetting and sharpness issues but to also allow chromatic aberration and purple fringe corrections based on the specific wavelengths of light that are misbehaving.
Even without shooting RAW or postprocessing images in a third-party noise-reducing software, photographers can plug their color-fringed photos into Photoshop for some fairly efficient fixes. Photoshop’s Lens Correction filter, in fact, has a chromatic aberration adjustment built in. It is operated by slider bars so you can see the effects of the changes as you dial them into the shot. Perfect for repairing chromatic aberration by eye.
For purple-fringe repairs, use just about any editing program’s Hue/Saturation controls and select only the magenta channel of the file. Dramatically reduce the saturation and adjust the lightness of the magenta channel to minimize the purple fringe. Further tweaks can replace purple with more appropriate hues, but simple reduction of saturation goes a long way. This approach has a major roadblock though: If large portions of the image contain magenta tones that you do want to retain, it’s best to work in layers and mask away the purple tones from the fringe areas that shouldn’t have them.
Because blue and purple fringes so often occur adjacent to bright blue skies, another common Photoshop fix is to select the blue sky and save the selection. Then expand the selection to include foreground element edges that contain the fringe, and finally subtract the original blue sky from the selection in the Load Selection dialogue. This approach leaves only the edges of image elements adjacent to the sky, so you can then adjust hue and saturation selectively without wholesale modification of the tones throughout the frame.
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