CA, color fringing and correction

Discussion in 'Image Processing' started by Klorenzo, Nov 4, 2015.

  1. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    I always thought that CA and colour fringing were closely related, like cause and effect.

    So I tried to remove some ugly purple borders with four different "CA correction" tools (gimp, rawtherapee, hugin, darktable) with absolutely no luck. Then I discovered the "defringe" correction and with a little tuning now the shot is perfect.

    I do not know much more than this, just wanted to save some time to someone else.
  2. barry13

    barry13 Super Moderator; Photon Wrangler Subscribing Member

    Mar 7, 2014
    Southern California
    Yes, defringe works well in RT, I just wish I could remember it's there as I always go for the CA sliders instead.

  3. David A

    David A Mu-43 All-Pro

    Sep 30, 2011
    Brisbane, Australia
    Both CA and fringing are actually chromatic aberrations, just different chromatic aberrations. Calling one "chromatic aberration" and the other something else can lead people to to think that CA is chromatic aberration and fringing is not but both are actually chromatic aberrations. CA is a lateral/latitudinal chromatic aberration caused by an inability to focus red, green and blue wavelengths at the same point, and gets worse towards the edges of the frame. Fringing is an axial or longitudinal chromatic aberration and occurs when some wavelengths are focussed in front of or just behind the field of focus. It can occur anywhere in the frame.

    So they are related, even closely related, because they both occur when wavelengths aren't focussed at the same point. It's just that in one case different wavelengths focus beside each other and in the other case they focus in front of/behind each other, hence. Each is caused by a different lens problem and you can have one without the other. Because of that, the fix for CA will not correct fringing and the fix for fringing will not correct CA.

    CA is the easiest to correct and that correction is even automated in many RAW converters these days. You just have to check a box to have CA automatically corrected in Lightroom and I leave that box checked as a default so I never have to think about it. I'm not aware of any fully automatic correction for fringing as yet but the correction is semi-automatic in LR. Don't know about in other programs.

    If your software allows for automatic CA correction, you can leave that option selected as a default. It shouldn't cause problems in images where CA does not occur. Any colour aberration you then see will almost certainly be fringing so you then use the defringe correction on it.
    • Informative Informative x 5
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  4. kwalsh

    kwalsh Mu-43 Top Veteran Subscribing Member

    Mar 3, 2012
    Baltimore, MD
    @David A@David A summarizes it just about perfectly.
  5. budeny

    budeny Mu-43 All-Pro

    Mar 4, 2014
    Boulder, CO
    This is not correct. It will cause weird artifacts where CA do not exists or do not cause any problems.


    CA correction enabled:
    View attachment 105168

    CA correction disabled:
    View attachment 105169
  6. kwalsh

    kwalsh Mu-43 Top Veteran Subscribing Member

    Mar 3, 2012
    Baltimore, MD
    To be clear, he is saying that leaving "Remove Chromatic Aberration" checked all the time by itself will not cause problems. And he's right about that. It is a lateral chromatic aberration correction and it pretty much never hurts anything and is fully automatic with no interaction from the user.

    In your example the problem you are having is because of the "Defringe" settings you have. Indeed, do not use Defringe in a preset! You better do it by hand for each photo and only use it when necessary. This is for longitudinal chromatic aberration and it can easily falsely detect fringing which is why so many sliders control it and it can be controlled with local adjustment brushes as well - it really needs to be fine tuned. And that's pretty much what David already says in his post as well...

    EDIT: To be clear the "Remove Chromatic Aberration" check box and the "Defringe" sliders are completely different controls and actions independent of each other. I think you may be confused thinking they are the same thing or related since in your example you changed all of them despite saying that one was CA enabled and the other disabled. That isn't what you did, you enable and disabled both CA and defringe in your example.
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  7. pdk42

    pdk42 One of the "Eh?" team

    Jan 11, 2013
    Leamington Spa, UK
    My experience is the same. Defringe can make an ugly mess at times. Never had a problem with CA correction though.
  8. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Same here: too much defringe (with Darktable) is going to blur/desaturate borders and seems to also desaturate a little the picture overall.
    With the right amount is just perfect. I also noticed that it can help a lot to remove residual chroma noise at high ISO.
  9. RnR

    RnR Mu-43 All-Pro

    Sep 25, 2011
    Brisbane, Australia
    He is such a showoff :p 
  10. David A

    David A Mu-43 All-Pro

    Sep 30, 2011
    Brisbane, Australia
    I agree :) 

    Actually I just went digging in Martin Evening's Lightroom book for the basics and paraphrased them. It's always handy having a few books around that you can refer to when questions like this one get raised because it saves having to use your memory. I can never remember which one of the 2 is lateral and which is axial without looking it up. Really I just wanted to clarify things in relation to the OP's comment about the 2 being related (they are) and that one caused the other which isn't the case. Not all relationships are causal.

    As for the Defringe correction, use it only when necessary. It can most definitely cause problems when there is no fringing and it can also cause problems when there is fringing. It's great at removing the colour of the fringe and that's a good thing if that colour is only present in the image in the places where you have finding. If the fringe colour is present elsewhere because something elsewhere in the scene has a colour in the range that the Defringe correction is removing, the correction will remove colour from that thing as well. Fringing is harder to correct than normal lateral CA and you are much more likely to see unwelcome problems trying to correct it than you will ever be with lateral CA correction.
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