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Using The GIMP for Chromatic Aberration Removal

Updated: 12/5/2005

Chromatic aberration (CA), also known as "purple fringing" can be a serious problem with digital cameras under certain conditions, such as a dark subject against a light background (like the sky). Tree branches are especially prone to this problem. Cyan, blue, and magenta fringing is common, with green fringing less so.

Here are a couple of examples of quick adjustments that can reduce CA using the GIMP , a free and open source image manipulation program.

Mike Larson

Here is a photo that was shot with a Panasonic DMC-FZ10 on an overcast day. The subject was dark and the background sky overexposed. This combination resulted in some serious chromatic aberration of the cyan/blue/magenta variety. 

This is the original image, cropped, downsized and compressed for the web. No other changes were made. There is magenta fringing on the underside of the limb and cyan fringing in the trees in the background.
Barred Owl

The corrections made to correct the CA in the original include (under Tools > Colors > Hue-Saturation)

Desaturate cyan (-60)
        "         blue  (-60)
        "         magenta (-100)

Other post-processing did not affect the CA, but included a levels adjustment, and slight color balance adjustments (red +10, blue -10), and sharpening. The final result is below.

Notes: 

1. The desaturation was done to the entire image. This is a very quick way to eliminate CA, but may have unintended consequences in other parts of the image. In my second example below, desaturating magenta caused problems with the red head of the woodpecker.

 Another option is to select only the fringe (possibly using the "select by color tool) and correct it without affecting the rest of the image. Obviously this is more time-consuming.

2. Too much desaturation may result in patchy looking areas where the are subtle changes in color. I try to use no more desaturation than necessary, in order to minimize problems.

3. The result of desaturation is to turn the cyan/blue/magenta fringe to gray. While not a perfect solution, gray is more natural in appearance than CA and will usually blend in well with branches.

4. The sky area of an image can be a difficult problem, since that is the high contrast area most subject to CA. Depending upon the exposure and actual sky color, the sky can range from a deep blue to gray. In between there is a cyan color which I personally find unattractive. Sometimes I desaturate the sky, preferring gray to cyan. I have also had success in increasing the cyan saturation to turn the sky more blue and "drowning out" the fringing. It all depends on  your photo and how much time you are willing to spend to get the look you want.
Barred Owl

Here is an example where corrections can not be applied to the entire image. Another FZ10 photo, this was taken on a bright sunny day, but the pileated woodpecker was on the shady side of the tree and I was looking into the sun. Lots of CA in this one. Purple in the lower right and cyan everywhere.
Pileated Woodpecker

After adjusting levels, the changes made to correct the CA in the original were:
Desaturate cyan (-100)
        "         blue  (-50)
        "         magenta (-100)

Red often has a strong magenta component and in this case desaturating it resulted gray patches in the red head and cheek of the woodpecker, as shown in the left image below. The solution was to desaturate the cyan and blue first. Then I selected the head, inverted the selection so that everything except the head was selected, and desaturated magenta. It was not necessary to select the head with care, because I was only going to desaturate magenta. All I needed was a quick loop around the head including all of the red. The result is on the right below.

Also notice that the desaturation of blue got rid of the blueish cast in the body of the woodpecker, making it a more accurate black.
Pileated Woodpecker

Here is the final result, after some sharpening.
Pileated Woodpecker