Remove Purple Color Fringing

One of my blog readers emailed me this Snowy Owl picture and wrote:

"Can you give any advice on how to prevent the purple coloring on some of my photos? This picture of a Snowy Owl has it really bad. It was taken with a Nikon Fieldscope III with 24x WA eyepiece."

Sadly, purple color fringing is an optical anomaly of the scope's glass and lenses of the camera, and cannot be prevented. But all hope is not lost. This purple color fringing, or chromatic aberration, is very common with the digiscoping technique. The purple glow comes from the inability of the lenses to focus all colors to the same point. It is especially noticeable in high contrast areas of an image, as this photograph clearly shows where white is adjacent to a darker background, the purple glow on the left side of the owl is characteristic chromatic aberration.

Using image processing software

The most effective way to eliminate chromatic aberration is to post-process it out using image processing software. A quick way to reduce purple fringing in an image is to desaturate the magenta color channel, but I usually target just the offending area with a series of steps.

Here's how I cleaned up the snowy owl image (Image A) with Adobe Photoshop:

  • With the image loaded, I used the lasso tool to select the purple area and created another layer via copy and paste. This separated the purple area into a unique layer where I could effectively work with just the offending part of the image.
  • Next, I used desaturate to remove all color data from Layer 1. This rendered a gray border along the left side of the owl. Keeping the focus on Layer 1, I used the selective color function to change colors to neutrals and adjusted the slide controls, changing the color of the gray area until it matched the background colors perfectly. Presto! The chromatic aberration is gone (Image B).
  • Additionally, I modified the levels and sharpened the image. I also used the sponge tool set to desaturate and make the owl's white stand out. This also took out some additional purple on the owl's body. Since the owl's eyes also had some purple, I used the lasso tool to select the eyes and created another layer on which I ran the selective color, bringing the eyes back to yellow. This last step used the same technique as above, but instead of matching the color to the background, I simply made the eyes yellow with the slide controls.

There are many other ways to remove chromatic aberration from digiscoped images, but this is the technique that has yielded the best results for my digiscoping work.

Article and photos contributed by Mike McDowell, an avid digiscoper, amateur naturalist, and Eagle Optics employee. Visit Mike's Birding and Digiscoping Blog.

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