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I know there are many tools that correct chromatic aberration like Lightroom, Capture One and even Photoshop camera raw, but I would like to remove it manually within Photoshop (no plug ins etc) -how can it be done?

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although simple edits are always the best, complex editing is very welcomed as long as it really does the trick -I like to put a strain on Photoshop... –  Pastel Mar 18 '13 at 14:32
    
Why wouldn't you use the lens correction filter in PS? It's built-in and does both forms according to lens profile information (depending on version). –  John Cavan Mar 19 '13 at 2:25
    
I haven't done it manually since the days when I still used scanned Velvia; lens profile information isn't available for such images. –  Nick Miners Mar 19 '13 at 8:58
    
@JohnCavan not every image I get/use has lens profile info -when I can I try to do things the easy way but its not always an option –  Pastel Mar 19 '13 at 19:02
    
It still offers the option of manually adjusting without the profile information. To me, that's the easy way... –  John Cavan Mar 19 '13 at 21:39

2 Answers 2

up vote 8 down vote accepted

I've done this myself in the past. If you look at the nature of the CA you should be able to work out an effective way to correct it.

If you see red/cyan fringing, e.g. black objects are fringed with red on the outside and cyan on the inside, then you can increase the size of the red channel slightly; it takes trial and error but it does work to some extent.

Equally, if the CA is green/magenta then you can try with the green channel instead.

To correct an individual channel, select the relevant channel in the Channels window so that it is the only one showing. Choose Edit > Select All, then Edit > Transform > Scale. Lock the aspect ratio of the image with the 'chain' icon, and enter a value such as 100.2% in the W or H field.

It's not the most accurate way to correct CA as it assumes the three colours are affected in isolation rather than as a continuum, but it can result in visible improvements.

Here is an example of CA at 100%:

CA Uncorrected

By increasing the size of the red and green channels by a fraction of a %, I get this result:

enter image description here

After you do it a few times, you'll get the hang of which channels you should adjust, and how much.

Note: this will only work for lateral chromatic aberration, and where the image is uncropped, so ensure you do this corrective step before cropping the image.

The reason it only works on uncropped images is that when scaling a channel, the centre of scaling is (by default) the centre of the image. In order to correct this form of CA accurately, the centre would need to be coincident with the axis of the lens, which will not be in the centre of the image if you have cropped it (unless you've cropped all edges by the same amount). You can try and work out where to scale from in a cropped image, but it won't be as accurate.

The other type of CA is longitudinal CA, which occurs when different frequencies of light have slightly different focal lengths, resulting in purple and/or green fringing. This effect can happen anywhere in an image, and is harder to correct as simply as lateral CA. See this question for more about longitudinal CA.

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(some j scripts were blocked before so I wasn't able to do many things including up voting & commenting etc) I like this idea a lot, it helps me better understand how channels work in general. you mentioned that it will only work for lateral chromatic aberration, and where the image is uncropped -1st why does it make a diff if i cropped the image already & 2nd what other types of CA exist? –  Pastel Mar 19 '13 at 19:13
    
@Pastel I've edited the question to answer your two follow-up comments - hope that helps. –  Nick Miners Mar 19 '13 at 19:21
    
thanks that really helps. maybe my next question should be how to fix longitudinal CA (btw i didn't even know different types of CA exist -thought it was all one rule, thanx for the tip) –  Pastel Mar 19 '13 at 19:29

I created a little action in Photoshop which does the following:

  • converts channels into monochrome layers
  • locks the green channel's layer (to preserve sharpness)
  • auto-aligns the three layers
  • merges the layers back into a color image.

Unfortunately it doesn't always work but when it does:

Before: before

After: after

You can download the .atn file here:

FixChromaticAberration.atn

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