Purple Fringe

Original Corrected

Chromatic aberration (CA), sometimes known as purple fringe (PF), is an unfortunate fact of life for modern-day digital cameras. Possible causes are discussed in my tutorial on Panorama Tools (click on Calibration > Color Fringing). The solution suggested involves a radial increase/decrease of individual color channels in an effort to bring features into alignment and eliminate the cause of fringing.

Frankly, I haven't had much success with this method. It assumes ideal textbook fringing that consists of complimentary primary colors on opposing sides of an object. To compound difficulties, fringing varies depending on light intensity and f/stop so determining coefficients to correct fringing can be problematic.

For a simpler approach select the offending colors in Photoshop and reduce saturation to alter fringing from bright purple to neutral gray. Select fringing with the Magic Wand, add a Hue/Saturation adjustment layer, and adjust saturation and lightness. Superior results may be obtained with the following variation:

• Duplicate, Blur, Color blend. Choose Ctrl-J to duplicate the Background layer, choose Filter > Blur > Gaussian Blur to blur the duplicated image, then change blending mode to Color.
• Select fringe colors. Select the Background layer and choose Select > Color Range. Click on the fringe with the Eyedropper, then choose the + Eyedropper, to add colors as needed.
• Invert and delete selection from blurred layer. Choose Ctrl-Shift-I to invert the selection, select the blurred layer, and press Del.

The purple fringe selected in the background layer is inverted and deleted from the blurred layer. This leaves the blurred layer mostly transparent with a few traces appearing where there was purple fringe.

The additional layer was blurred so that surrounding colors would invade the fringe areas and yield more natural results under a Color blend. When blur radius and fringe colors are properly calibrated the procedure is very effective.

A Purple Fringe action is included in the Actions download. Each camera/lens combination has it's own unique fringe colors, so you'll need to modify the action to match the characteristics of your setup. The following steps are recommended.

• Open a particularly obnoxious image.
• Double-click on the Color Range step in the action.
• Use the Eyedropper to select the first color.
• Add additional colors with the + Eyedropper.
• Enable the dialog box for the gaussian blur step and run the action.
• Include just enough blur to accommodate your worst-case sample.

This technique was first developed by Shay Stephens, and it does an excellent job at removing purple fringe while retaining image tonality. Actions for both Photoshop and Paint Shop Pro are available at Shay's web site.