Insidious Chromatic Aberration - Help

[plinden]

Hi, I would appreciate your comments and suggestions on a problem I'm having with CA. I shoot landscapes with both an H1/Kodak system as well as a 5D. Along the way I guess I encounter a "normal" amount of CA (see exhibit A). While using the CA sliders in ACR helps here it doesn't fix it to my satisfaction.

However, lately I have noticed a more obnoxious type that seems to vary by the surrounding colors and it is expecially hard to fix. Exhibit B shows a 5D image where the CA blends with the sky color. In exhibit C the same thing happens in an H1 image but the CA takes on both a blue cast that mixes with the sky or a red cast that mixes with the yellow foliage.

I read of a "purple fringe" technique that does a color based selection and desaturation. This doesn't work when the surrounding area is the same general color as the CA!

Can you offer any suggestions to cope with this?

Thank you.

Peter Linden

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[TimothyFarrar]

One thing I have used to remove messy CA in images is by creating a merged copy of the image and running a 3.0 pixel radius gausian blur, then starting with a black layer mask, paint with a white brush in the layer mask on the areas with CA. This can also be used to spot correct problems with very high contrast single pixel edges which have Bayer grid interpolation problems.

CA on bright edges (such as a horizon) can require a different approach, sometimes using a 2.0 pixel radius minimum filter followed by a 2.0 pixel radius gausian blur can select the darker color and remove the bright edge CA (use the same paint in the layer mask approach as above).

Either way, its not a solution for batch processing...

[colourperfect]

Do you have some 100% or 200% crops ? I must admit its difficult to distinquish CA from jpeg artifacts or possibly sharpening.

Ian

http://www.colourperfect.co.uk

Hi, I would appreciate your comments and suggestions on a problem I'm having with CA. I shoot landscapes with both an H1/Kodak system as well as a 5D. Along the way I guess I encounter a "normal" amount of CA (see exhibit A). While using the CA sliders in ACR helps here it doesn't fix it to my satisfaction.

However, lately I have noticed a more obnoxious type that seems to vary by the surrounding colors and it is expecially hard to fix. Exhibit B shows a 5D image where the CA blends with the sky color. In exhibit C the same thing happens in an H1 image but the CA takes on both a blue cast that mixes with the sky or a red cast that mixes with the yellow foliage.

I read of a "purple fringe" technique that does a color based selection and desaturation. This doesn't work when the surrounding area is the same general color as the CA!

Can you offer any suggestions to cope with this?

Thank you.

Peter Linden

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]
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[plinden]

Do you have some 100% or 200% crops ? I must admit its difficult to distinquish CA from jpeg artifacts or possibly sharpening.

Ian

http://www.colourperfect.co.uk
[a href=\"index.php?act=findpost&pid=60191\"][{POST_SNAPBACK}][/a]

Ian, if you click on the images you will get a 100% crop. All these images were originally shot raw so there were no artifacts at that point only on final conversion to post. These posted images pretty well show what I see in raw. Also I am normally very conservative in sharpening (at native file sizes a=300, r=0.3, th=0).

Peter

[Jonathan Wienke]

There are 2 types of CA, one of which is easily fixed in ACR (where complementary colors fringes are found on opposite sides of an object) and one which is not (a single-color fringe goes all the way around an object). Here's my technique for fixing the "difficult" type of CA:

Convert to 16-bit mode if you aren't already.

Convert the image to LAB mode.

Select the A and B channels, by clicking on A in the Channels palette, then shift-clicking B. You should see a rather weird rendering of your image with everything at 50% luminance, but the color fringes should be clearly visible.

Run the Dust & Scratches filter at whatever settings are necessary to get rid of the color fringes.

Go back to the Channels palette and select LAB channel.

Go to the History palette and select the bottom (last) history state as the source for the History brush. (Look for the little box on the left of the history state, and click it to make a little brush icon appear.) Select the next-to-last history state, which will undo the effect of the Dust & Scratches filter.

Using the History brush, paint out all of the objectionable color fringes in the image.

[Dmitry]

Corel Paint Shop Pro 9 and X have filter for correcting Cromatic Aberration and Purple fringing.

[Lin Evans]

Hi Peter,

Actually a good action such as the one you can purchase from Shay Stephens:

http://www.shaystephens.com/

will handle all three of these issues. The third example (on the right of the screen) is difficult to see without an LCD monitor, but Shay's manual action lets you move the slider when selecting the color fringe you want to remove and lets you see the results before you save (uses layers) so you can effectively "undo" your work if it interferes with other colors.

In all three of these cases the action successfully removed the offending colors without seriously affecting the remaining image.

Lin

[david75]

You can use this plugin for correct CA: http://epaperpress.com/ptlens/


Bye,