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Old Jan 17, 2004, 11:35 PM   #1
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Default chromatic abberation

hello.. can purple fringing be edited out cleanly and effectively using post production software?

if so, please provide links.. thanks!
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Old Jan 18, 2004, 8:20 AM   #2
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What software are you using?

A little searching on google turned up this photoshop action:

I don't know how well it works, though.

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Old Jan 18, 2004, 3:16 PM   #3
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CA cannot really be 'edited out'. The best you can do is try to hide it, which is why I think it's one of those compact digicam lens distortions that should be designed out or minimised from the start.

Th usual technique with Photoshop is to change the purple edge to a shade of grey with the colour picker, which doesn't get rid of it, but makes it less noticeable. The problem here is you pay a lot for Mpix and clean sharpening to give you edge detail, then throw it all away with a lens poor on CA.

If you want to see the effect, just convert some images to Red/Green and Blue Channels (viewed in monochrome), then compare edges on the blue channel with green (nearly only luminance info) or do a subtraction. Cheers VOX
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Old Jan 21, 2004, 9:09 PM   #4
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Picture Window Pro ver 3.5 has a CA removal tool that works pretty good.

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Old Jan 21, 2004, 10:53 PM   #5
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It depends on the "cause" of the purple fringe. True chromatic aberrations are lens centric - that is when light is passed through a medium like glass, it's sometimes broken into the various color components which comprise white light (think of how light acts when a prism is used).

Optical glass in lenses is treated with different types of coatings and there are multiple elements in most good lenses which together attempt to keep the light coherent as it strikes the film or sensor. The better glass is called "low dispersion" glass and sometimes APO or other names are used which reflect the various processes which "correct" the tendency to create classic chromatic aberration.

Without getting too technical, there are several types of chromatic aberration but in general it's seen as either blue or red/green fringe and most usually in the peripheral areas of a lens.

Another type of fringe color comes from sensor blooming where electrons spill from one photosite to adjacent photosites when the photosite (pixel) is oversaturated from excessive light. This "fringe" often take one or more shades of purple or violet and is usually seen in areas of very high contrast such as back-lit subjects (think leaves or tree limbs against a very light or intense sky).

Sometimes blooming appears to exacerbate chromatic aberration and a single solution is not possible for correction.

The red/green and blue classic chromatic aberration can often be greatly improved with software which differentially affects the RBG images which comprise the channels and together make the image. By actually shifting the position and changing the size of the revelant chroma signals, these chromatic aberrations can be greatly ameliorated or sometimes made to disappear entirely.

AS mentioned in the post above, Picture Windows Pro 3.1 and later offers a great tool which works very well. Lesser degrees of "fix" can be found in programs like Qimage, and the latest version of PhotoShop also has chromatic aberration tools.

The blue/purple/violet fringe from blooming or from a combination of blooming and localized CA can't be controlled this way. There are two approaches which do work pretty well. The first has already been mentioned where PhotoShop tools like "replace color" can be used to "pick" the offending hue and desaturate leaving a grey rather than purple border. This sometimes works well and sometimes dosn't depending on the individual circumstance. It's also possible to use the "masking" tools in PhotoShop or third party software to localize the correction so that should there be this identical hue elsewhere in the image where you "don't" want to remove it, the action is limited to the area masked.

Just as one can "desaturate" the offending area, it's also possible using the "replace color" picker to change the purple/blue/violet hue to a different color. Sometimes this works much better such as when the fringe is bordered by blue sky. Instead of making the fringe "grey," you simply shift the hue to match the sky and it becomes invisible. There is no single treatment which works best for every situation, and no single PhotoShop "action" which can effectively help in all cases. Experience and practice are really the only way to approach correction of blooming and non classic CA.

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Old Jan 22, 2004, 1:11 PM   #6
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Thanks for that writeup. After your paragraph about red/gree or blue CA, I thought "what about purple?" and then you covered that too.

That was helpful, thanks.

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Old Jan 22, 2004, 6:05 PM   #7
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cheers Lin!
thanks for the helpful info

i'll give Window Pro a try anyway, and i guess i'll have to get off my lazy a$$ and learn how to do photoshop tweaks.
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