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10/04/2011

How to remove purple-cyan fringing in The Gimp

(Check out the 2nd part of this tutorial here)

So, you’ve been struck by purple fringing… What’s this ?

When you take a shot with a CCD camera, zones with high-contrast (for example, the leaves of a tree with the sky as the background) may appear with a purple and green halo.

There are 3 ways to avoid this:

  • Use a better lens, with no chromatic aberration. Not always possible. Prime lenses have nearly no aberration since they are very well tuned for their focal length.
  • Use a UV filter. I’ve read this is a solution. I’ve not tried it myself, yet. I don’t have any filter, though this is in my to-buy list.
  • Use a software to correct it.

Oddly enough, my D7000 corrects the aberration on JPEGs, but does not apply this correction to NEFs (Nikon’s RAW file format). Since NEFs are supposed to be out-of-the-sensor images, with no corrections applied, this makes sense.

The NEF file converted by UFRawThe JPG file from the D7000

So, you have an image with CA (Chromatic Aberration) and you want to correct this with The Gimp. Quite easy:

  • Load the image into The Gimp.
  • Select the zone you want to correct. The tighter the selection is, the better. You may end up applying the correction to zones you don’t want to.
  • Select Tools > Hue-Saturation.
  • Here comes the trick. Select the magenta option, and desaturate it (around −70% will do, YMMV).
  • Apply and select the menu again.
  • Now select the cyan option and do the same, desaturate.
  • There you go.

The results:

The NEF file converted by UFRaw
The same as above
The file processed with The Gimp

In this particular example I’ve removed Magenta, Cyan and Green fringing.

This technique does not work 100% in all the cases, but it has really helped me a lot in some situations.


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