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Here is a zoomed-in example of an image I shot that has a bit of discoloration due to the chromatic aberration of the lens.

aberration

What's the easiest way to fix this problem using only free software such as GIMP? I'd like to for the method to preserve color in other parts of the image too.

Note that there are other questions about fixing chromatic aberration, but none of them address methods of doing it with free software.

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Darktable screenshot

Darktable can remove chromatic aberrations if you are shooting raw. Your example image is a JPEG so I can't demonstrate how it works with that, but above is a screenshot of it removing the chromatic aberrations from a photo of a table lamp.

In Darktable this is done with the 'Chromatic aberrations' module available in the right pane, part of the correction group. It is a simple on/off toggle - no adjustments are needed.

Darktable is free and open-source software and I highly recommend it.

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Rawtherapee can do it to processed images. For some reason, "Defringe" is located on the detail tab, and you can specify what colors you want to take care of.

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  • \$\begingroup\$ fringes and CA are different effects. CA can be removed by scaling the three colour channels differently. fringing needs destructive methods, eg desaturating. \$\endgroup\$
    – ths
    Nov 9, 2014 at 10:43
  • \$\begingroup\$ Agreed. RT has true CA for raw processing, but only fringe reduction for rendered images. However, if you want to get technical about it, scaling the channels is destructive as well. \$\endgroup\$
    – JenSCDC
    Nov 10, 2014 at 9:05
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You can use Lensfun. Lensfun is a free/open-source (LGPL) library for fixing a number of defects from lenses.

Check the list of supported lenses here: Lensfun coverage. See the "TCA" column, they are the lenses where data is available for correcting chromatic aberration. Note this is currently only a small number of lenses. If your lens is not supported, you can upload sample photos, to allow this to be calculated, or do the calculations yourself. See Upload calibration pictures.

Lensfun is just a library, so you need an application to run it. An easy and free way is to use GIMP, with the GimpLensFun plugin.

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