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Removing Chromatic Aberration using Photoshop

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Frankman View Drop Down
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Post Options Post Options   Quote Frankman Quote  Post ReplyReply Direct Link To This Post Topic: Removing Chromatic Aberration using Photoshop
    Posted: 25 November 2008 at 12:00
We all love those old Minolta lenses. The colours are wonderful, however, being designed long before digital, the coatings don’t suppress Chromatic Aberration (CA) very well. We’ve all seen chromatic aberration in our shots. It’s that horrible purple, blue, green or red halo that occurs in regions of high contrast on our images. There are a number of ways of removing CA. I will describe the method I use, which I’ve found to be far superior to most other methods. No matter whether you use old glass or new glass, regardless of how expensive the glass, you will get CA in some shots.

If you look at the following image, you’ll see some horrible purple fringing at the leading edge of the bird’s wing, where black meets white.




In Photoshop, create an adjustment layer: In the layers palette, click on the adjustment layer button at the bottom of the window. Then click on Hue Saturation. Lookie here:



A window will open like this:




From the drop down list where you see “master”, select “Magentas”. Then move the “Saturation” slider down to around -50, and move the “Lightness” slider up to +50 (no need to be exact). Then, if you look down along the bottom of the window, there’s a little slider that you can drag along the “rainbow” until your CA disappears. Magic!

Now, if you’re observant, you will have noticed that there’s a bit of green CA on the back wing – look near the left edge of the image. To remove this artefact, simply repeat the process. Open a new adjustment hue/saturation layer. This time choose greens instead of magentas. Then just repeat the process.



How easy was that! With a bit of practice, you should be able to do this in less than a minute. The result is certainly worth the effort. A lot cheaper than buying new glass too!

Hope you find this technique useful.

Frank


Thanks to Frankman for volunteering this article. It is likely that the technique employed can be adapted for use with other PP programs such as Paint Shop Pro. Feedback to this effect is welcomed.


POSTSCRIPT

A few follow up comments to this article have queried how to overcome the issue of desaturating other parts of the image that you don't want to desaturate.

I delete the hue/saturation layer mast, then add an inverted layer mask (hold down "alt" when you click on the "add layer mask" icon. You'll notice that the layer mask icon is now black, which means "reveal all". Use a white brush (on the layer mask) to go over the areas that are affected by CA. This will restrict the hugh/saturation to these areas only, leaving other areas unaffected.

Hope this makes sense.

Frank


Edited by Frankman - 26 November 2008 at 07:04
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RosieA100 View Drop Down
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Post Options Post Options   Quote RosieA100 Quote  Post ReplyReply Direct Link To This Post Posted: 25 November 2008 at 14:23
I had a go at this earlier today and it really works (as if it wouldn't)!!! Thanks so much Frank for putting this in simple language so I could understand
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DavidB View Drop Down
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Post Options Post Options   Quote DavidB Quote  Post ReplyReply Direct Link To This Post Posted: 25 November 2008 at 14:28
Wow! Thank you for this Frankman, I will definitely be using it on some of my work.
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Post Options Post Options   Quote momech Quote  Post ReplyReply Direct Link To This Post Posted: 25 November 2008 at 14:53
Thanks Frank. I've tried it a couple of other eays; this looks simpler and very effective.
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Mark L View Drop Down
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Post Options Post Options   Quote Mark L Quote  Post ReplyReply Direct Link To This Post Posted: 25 November 2008 at 18:39
There is the possibility that the hue/saturation adjustment layers will affect colours elsewhere in the image -- many images contain greens or magentas that you would want to keep.

The solution to this is to create a duplicate of the background layer and move it (left button and drag) above the adjustment layers. Then use the eraser tool to remove the areas where chromatic abberation shows, to reveal the same areas in the adjusted background layer underneath.
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Post Options Post Options   Quote outback88 Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 02:27
It would be great if other forum members who use different S/W such as bibble, gimp, rawhide, IDS & others, could also add their CA removal remedies, that way we have everyone covered, l don't use PS but l have a beercan & CA is a problem, l use IDC or Raw Therapee, l don't know how to remove CA in either of these.
 



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Post Options Post Options   Quote polyglot Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 03:17
Hey guys, just a nitpick, but CA has nothing to do with coatings; it's a lens-design issue wherein the image magnification is wavelength-dependent. Modern lenses like the 16-80ZA have huge CA.

The best way to fix lateral CA is in the RAW processing stage, where you scale the three colour channels independently so that the image magnifications in each channel come out the same. Result is no CA and more sharpness without damaging the colours in other parts of the image or needing to futz around with masking.

Axial CA is more difficult (and still present in modern lenses; the Zeiss 85 and 135 both show it a lot) because it's a matter of different wavelengths focusing behind or in front of the sensor plane, so bright/dark things are uniformly surrounded by a colour depending on which side of the DOF they are. Can't be fixed by scaling the colour channels independently, so Frankman's technique is probably good for axial CA.

Chromatic Aberration reference.
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Post Options Post Options   Quote wetapunga Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 03:19
Originally posted by polyglot polyglot wrote:

...
The best way to fix lateral CA is in the RAW processing stage, where you scale the three colour channels independently so that the image magnifications in each channel come out the same. ...


Hmm, please explain this 'scaling' approach.

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Post Options Post Options   Quote dennismullen Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 03:34
I agree the best way to fix CA is to shoot Raw and use the lens corrections tool in ACR. There is a tool in PS too but it messes with the edges of the picture.
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Post Options Post Options   Quote sybersitizen Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 03:41
You don't need to shoot RAW for effective CA correction - you don't even need Photoshop. PTLens handles that task and a lot more; it works as a Photoshop plug-in or standalone program; and costs just $25 for a perpetual license.
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Post Options Post Options   Quote polyglot Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 03:41
By scaling, I mean image-scaling. You know, a resize, but applying a different resize factor to each of the R, G and B channels to account for the different magnifications that the lens produces for each colour.

It's not something I've ever attempted manually (and it would be really difficult manually) and ufraw doesn't seem to support it , but there's a "CA correction" option in some RAW converters, at least PS CS and ACR and probably some others too, that will do it for you.

And this is for lateral CA only, but that's the common sort. Axial CA is typically seen only with large apertures, like f/2 or more, and for some reason is a bit less objectionable, at least to me.

edit: thankyou sybersitizen for posting the canonical tool for this!

Edited by polyglot - 26 November 2008 at 03:43
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Post Options Post Options   Quote madecov Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 04:01
I'm a real novice at RAW, Have not used layers. I don't have a work flow to speak of.
I downloaded a CA "filter" plug and have played with it. It worked well in the one image I played with that was shot with a Minolta 24-85.

This is the original done in IDC


Then I used tweaked it a bit more and used a plug in called PF FREE
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sybersitizen View Drop Down
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Post Options Post Options   Quote sybersitizen Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 04:47
Originally posted by madecov madecov wrote:

... used a plug in called PF FREE

Actually, PFree (if that's the plug-in you mean) is designed to deal with purple fringing, which is a separate phenomenon from frankman's axial CA, which is a separate phenomenon from polyglot's lateral CA!

Purple fringing and axial CA are dealt with by subtracting or desaturating particular colors, but the problem with that is that other matching colors that belong in the scene may also be affected. For example, I can see that the purple flowers in madecov's photo have lost their intensity along with the unwanted purple tinge in the grave marker.

Lateral CA is handled mathematically by rescaling the color channels as polyglot described, so it can usually be fixed without undesirable side effects.

Edited by sybersitizen - 26 November 2008 at 04:50
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Post Options Post Options   Quote wolfy Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2008 at 05:18
Well I just learnt another photoshop trick I didnt know, if this keeps up I might just learn how to PP properly. Thanks Frank!
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