How to use frequency separation to remove chromatic aberration and colour fringing

Frequency separation is typically seen as a technique for retouching skin – albeit often quite badly these days. But that’s not its only use. Separating colour from detail offers a lot of other potential benefits for working on your images.

In this particular example, from travel & landscape photographer Michael Breitung, it’s chromatic aberration and colour fringing that get the frequency separation treatment.

No matter how good our lenses, chromatic aberration and colour fringing can still creep into our shots occasionally, especially as we move towards the edge of the frame at wider apertures.

The basic process to separate the “frequencies” is basically the same as it is when retouching a portrait.

Duplicate your image layer twice. Name one “Low” and one “High” (hide the “High” layer)
Select the “Low” layer and run your desired blur tool. Michael uses Gaussian blur
Blur it just enough to hide the purple fringing in contrasty areas and hit OK
Turn on the “High” layer, select it and choose Apply Image from the Image menu
If you’re working on a 16Bit image, use the following settings…

If you’re working on an 8-Bit image, uncheck “Invert” on the source channel, change the blending mode to subtract, and the offset to 128
Then hit OK
Change the “High” layer blending mode to Linear Light

You can see in the screenshot above that some of the purple colour fringes are actually in the detail layer now. This allows you to change them separately from the colour in the rest of the image. As you recolour the purple parts of the High layer over to grey, you’ll start to see the colour fringing disappear.

It’s worth noting, though, that this does destroy colour information. In very highly detailed, colourful images, other colours can creep into this layer, too. So, it’s not going to work for everything. But you can always add some colour back into the “Low” frequency layer, too.

For a more detailed look at frequency separation, check out this video from Conny Walstrom. It can be a very versatile tool, depending on how you use it. I don’t often use it for people, but I use it for animals all the time. I find it great for cleaning up fur and feathers.

[via ISO1200]