Chromatic aberration artifacts in Lightroom 6/CC 2015

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The CA (chromatic aberration) removal in Lightroom CC 2015 / Lightroom 6 sometimes creates weird artifacts -- see https://flic.kr/p/svaC3g

At the very least, turning off chromatic aberration correction should actually turn it off. I had to set the sliders to zero or disable the lens corrections module.
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gatoatigrado

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Posted 3 years ago

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LRuserXY

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"remove chromatic aberrations" and "defringe" are two completely independent features. Turning off "remove chromatic aberrations" does _not_ turn off defringe. Defringe can only be turned off by setting both "amount" sliders to zero.

EDIT: I checked your example and came to the conclusion that the behaviour is normal and intended. The complete background of your image is slightly blue. The first two sliders for the purple hue instruct LR to search for a color "band" of that size and color range, and the background blue fits those settings, so LR detects "blue fringes" around the letters and tries to compensate (within the "band size" given by the "amount" slider) using an overlay of an complementary color, which is reddish/pinkish in your case. This is how the manual defringe controls work - and these are simply the wrong settings for your photo (in fact, the area shown is not a candidate at all for defringe). Note that for a given defringe setting that works on one part of the image, there are often image areas for which these settings are wrong, and they have to be masked out using the correction brush.

TL;DR: Not a problem, everything works as expected (IMHO).
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LRuserXY

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Correction: The part containing "overlay of an complementary color" is not correct. See my other comment: Defringe seems to use the color of the area that is _not_ corrected to color the area that _is_ corrected, in this case red/pink (from the 1-px-edges around the letters).
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nick tung

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> "remove chromatic aberrations" and "defringe" are two completely independent features. Turning off "remove chromatic aberrations" does _not_ turn off defringe. Defringe can only be turned off by setting both "amount" sliders to zero.

that doesn't sound right, can an Adobe rep comment? "chromatic aberration" is another term for "color fringing", so it'd make sense that a "defringe" slider would change it. cf http://www.gizmag.com/remove-chromati...

> EDIT: I checked your example and came to the conclusion that the behaviour is normal and intended.

IMHO describing introduction of horrible artifacts as "normal and intended" is unhelpful. I do agree that all correction algorithms have some margin of error, and I was probably not controlling that as much as I should have with my current settings of the "purple hue" and "green hue" sliders.

Thanks for mentioning the local adjustments tool's defringe option. However, I still think as things stand this is still a bug, I think it should be smart enough not to introduce such artifacts.
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LRuserXY

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> IMHO describing introduction of horrible artifacts as "normal and intended" is unhelpful

Unhelpful? Really? I tried to explain why the artifacts are completely normal when using inappropriate defringe settings. How is that unhelpful?

> However, I still think as things stand this is still a bug (...)

Defringe is a semi-automatic correction that largely relies on manual input for strength and desired color range. With current technology it is impossible to analyze whether a color edge is caused by LoCAs / purple fringing or is correctly a part of the photo's content. So if the manual input is off, then the resulting image will contain artifacts. Especially if you set a color range slider so that the background color is selected, defringe will treat the whole background as a color fringe and try to counteract it. In the sample image, there seem to be slight red color edges around the letters, so the defringe correction wil use that color as the target color to replace the blue background with. The result is a thick red/pinkish color edge around the letters - and its size is limited by the amount slider.

Note that it should be possible to correct the red color edges around the letters - just set the purple hue slider so that it covers the red/orange colors (e.g. 50...100), and the red borders should disappear without any artifacts. You can also use the "fringe color selector" tool, but even in 8:1-zoom this could be difficult to use here because of the extremely small color edges.

> (...) I think it should be smart enough not to introduce such artifacts.

Of course it is desirable to have a completely automatic correction that does not require manual input and/or does not produce such artifacts. But that would be a feature request, not a problem report. The present version of the defringe tool works "as advertised".
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Steve Sprengel, Champion

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There are two types of CA:


The Chromatic Aberration checkbox helps remove Lateral CA, which is caused by different sizes of the red, green and blue images on the sensor. This increases from the central point of the image outward and can be modelled and corrected for using the image, itself.

Axial CA also leads to fringes anywhere on the image dependent on the distance in front of or behind the focal plan. This cannot be modelled and cannot be corrected for automatically because various parts of the subject and scene can be behind and in front of the focal plane arbitrarily.

Correcting this arbitrary Axial CA and other fringing is what the Defringe slider is for.

The type of correction Defringe does is based on desaturating the selected color range along an edge whether or not the color is caused by Axial CA or not. Also increasing the strength of the correction can overcorrect and the desaturation can "bleed" into the main image area away from the actual fringe.
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LRuserXY

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See also this explanation with examples: http://blogs.adobe.com/lightroomjourn...
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nick tung

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Thanks both. I agree with LRuserXY there is a theoretical issues of determining what is image content and what is fringing. I guess my frustration here relates to the edge in question being most black and white, so the defringe operator making something more saturated (towards red) was unfortunate.

Also, for context, part of what I was trying to do was create a lens profile that removes fringing in one click. I guess this isn't possible with current technology, and I'll have to tune it for each image. Anyway, thanks for the defringe/lateral ca clarification
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Steve Sprengel, Champion

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The Adjustment Brush, Radial Filter and Graduated Filter all have a Defringe component for when image areas match the fringe characteristics.

The scenario I commonly use the AB-DF for is a wide shot of a crowd in overhead lighting or windows where someone's clothing colors invariably matches the fringe color along the bright edges.
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Steve Sprengel, Champion

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Also defringing should be desaturating the edges not turning them reddish. If that is happening, something may be wrong with your display profile.
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LRuserXY

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I get the same result on my calibrated monitor, so I think it's really the very small reddish edge around the letters causing this (see my above reply). Eric Chan wrote in the lightroom journal article regarding the cake example: "Note that this is not a straightforward desaturation (which would turn the letters and cake gray)". Because of that I think that defringe paints over the "offending" color using color samples from just outside of the fringe. In the example with the letters, this is only the reddish edge (the other side is continuously blue, so the fringing doesn't really end), which leads to the reddish artifacts.

At first I thought too that something must be wrong here (either display related or a bug in the defringe function). But then I tried it on some sample pictures myself and got the same effect in similar situations: Black details with a little red fringing before a light blue background, with defringe incorrectly set to capture blue tones results in a large bleeding of the red into the blue background.

HOWEVER (!): I also found instances where the same happens _without_ the red fringing around the black details. That is really strange, so perhaps there is something wrong with defringe after all - I am not so sure anymore ;-) ... but anyway: this does not really matter IMHO because setting defringe to capture blue on a blue background is not a correct setting in the first place....
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Steve Sprengel, Champion

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In the OP's example, the blue is being defringed. It's hard to judge what is happening without the original image to experiment with.