Friday, March 19, 2010

Removing Chromatic Aberration

Q: "What is that purple glow in my digiscoped bird images?"

The purple color anomaly along the right side of this Barred Owl is an artifact that occurs when the optical system fails to focus all colors to the same convergence point. It can appear visually when viewing through a binocular or spotting scope, as well as a photo, and is typically exhibited along terminating edges of contrast areas. It's especially prevalent in digiscoped photographs. Technically known as Chromatic Aberration, this purple color fringing threatens to diminish the aesthetic appeal of your hard won photographic efforts.

Here's a quick & easy way that I remove Chromatic Aberration from digiscoped images using Adobe Photoshop CS. For demonstration purposes, I've cropped the original image to where the greatest amount of Chromatic Aberration is present.

To remove the aberration, we'll use Photoshop's "Replace Color" adjustment, which can be found from the menu: Image -> Adjustments -> Replace Color. This will bring up a window that looks like this (click on image for larger version):

Using the eye-dropper, make a color selection from the purple area. Repeat this while holding down the SHIFT-KEY, sampling and adding a variety of purple hues to the selection. As you do this, you'll notice that the black and white selection image becomes more defined to match the complete purple area. Once you're done selecting purple hues, move the Fuzziness slide control to the left until the area you want to fix is completely white and everything else is black:

In the final step, adjust the Replacement Hue, Saturation, and Lightness slide controls until the selected area (previously purple) matches the rest of the bird as close as you're able to:

Click [OK] to apply the changes, and presto! No more annoying purple color fringing!

Mike McDowell
Eagle Optics Staff

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