How to deal with chromatic aberrations

(Olympus 5050)

 
by Philipp Sanke © 2004
(with help from Jens Birch, Peter Lindsay, Alfred Molon among others)

 

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I. Do you have to deal with it at all?

First you should ask yourself: Do I really need to deal with it? Depending on your shooting style and on your motives maybe there aren’t any chromatic aberrations (CA) - visible as coloured fringes - in your pictures. Or they have some CA, but you just want to have normal sized prints from your pictures. Then don’t zoom those pics up to 400% in your Photoshop - just forget about it, make your prints and be happy.

Nobody ever will notice unless he is equipped with a magnifying glass.

 

II. Better than post-processing: Avoid it!

You want to do larger sizes prints - but are not willing to do any CA-correction on your pics? Try to avoid CA while taking your picture. This might help:
 

III. You want to or have to deal with it?

You want large prints? You have shot or plan to shoot bright, high-contrast motives with maximum wide angle? There is or will be CA in your pictures?

Okay. Here’s what you can do to get the best results possible. The options described below are based on thorough testing:

  1. Best method: Shoot in RAW. Import through Photoshop CS using the Adobe Raw Import-dialog. Magnify a corner of the pic in the preview window with CA visible. Correct CA using the two sliders provided for this. You’ll get almost perfect results - and it takes only some seconds to achieve them. CA disappears almost completely.

  2. Second best method: Shoot in RAW. Import through Photoshop 7.0.1 and Adobe Raw Plugin 1.0, through Camedia Master or through any picture editing software that supports the free Olympus RAW-plugin. Convert your imported picture to 8 bit per channel.

    Then correct CA with the plug in "Debarrelizer" from www.theimagingfactory.com (sells for 39US-$, free 30 day-trial available); this can be used with any editing software that supports Photoshop plug ins, e.g. PaintShopPro, Corel Photopaint and lot’s of others.

    Debarrelizer provides a preview window and the same two CA-corrections sliders as the RAW Import-dialog of Photoshop CS. You can reduce CA, but the results are not as good as with Photoshop CS.

    Instead of using the commercial plug in you can try more complicated, but free methods: Peter Lindsay suggests enlarging the green channel of the picture with a starting value of 100,05% both vertical and horizontal- also the blue channel if necessary. Experiment with these values. CA can be reduced with this if you have freshly imported RAW data. I won’t elaborate on PanoTools here, but one should be able to do something similar with them.

  3. Third best method: Shoot in uncompressed TIFF. In post processing use Debarrelizer or comparable methods as described in (2). The results are not as good as in (1) and (2) with RAW data.

  4. Worst case: You’ve shot in JPG? You can try Debarrelizer or comparable methods described in (2), and may be successful with that - but chances are that you’ll be out of luck and won’t get any useable results. The reason behind this seems to be JPG artefacts in some of the RGB channels (the blue one is the worst in most of my test pictures), even if in-camera sharpening is set to -4 or -5. If the channels have different amounts of artefacts, you can’t correct CA with the radial shifting Debarrelizer or comparable methods provide. What can you do instead? Check out the next section!


 

IV. No luck so far? Try this!

You haven’t got useable results so far? No plug in helped? Got JPGs with bad CA? Or do you have something which looks similar to CA, but really is something different: purple fringes at the edges of very bright, overexposed areas? Let’s call that "PF" for purple fringing. There is no possibility to correct PF through the Methods described in Section III. You may even have a dreadful combination of CA and PF...

In any case: Don’t despair! You just have to switch techniques. Desaturation is the keyword here. I’m afraid that most of the methods listed in the following are based on using Photoshop.

  1. If you got fringes mainly of one type of colour try this simple method described here: http://www.outdooreyes.com/photo26.php3

  2. If you got fringes of other colours (green, blue, yellow to mention just a few one), use method No. 1, but change the colour range (chose from the dropdown on top of the dialog). Then use the colour selection pipettes on the saturation dialog to pinpoint the colour range you want. It’s a good idea to use correction layers - one for each colour.

  3. If you want it comfortable, check out the Photoshop and Paint Shop Pro actions from www.shaystephens.com. They’ll cost you 10 Dollars, but they’re easy to apply and work fine (at least the Photoshop version does, I didn’t test the other one).

The problem with all methods described above: They work on the symptoms, not on the cause of CA (with PF you have no choice but to work on the symptoms). Because of that there is a little loss of detail and sharpness involved.

Also if you have areas in your picture that have colours close to the colours of the fringes you are working on, they are also affected by desaturating. E.g. someone wearing a purple shirt may get a grey one if you desaturate purple. Grass and plants may become grey if you desaturate greens. There are a number of remedys for that:

Shay Stephens actions (3) create correction layers for each colour that can be corrected simply using the eraser tool.



V. Conclusions

Go on, take pictures! If CA does crop up, use this guide.

If you discover errors in this text or have additional suggestions, drop me a line (email to: immo@justjog.de).

Yours!

Phil