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Thread: CAs in BW conversion

  1. #1

    Join Date
    Nov 2011
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    Ian

    CAs in BW conversion

    I'm using LR3, but have been trying to use PS CS5.1 to sort a particular problem I've come across.

    After converting to BW and making basic exposure adjustments, tone curve adjustments, etc, I've been using the B&W mix sliders to make final adjustments to tones, particularly applying a negative adjustment to blue to darken the sky. This produces an unwanted side effect on dark objects against the sky, so tree branches and birds, for example, acquire a light border, which seems to be caused by Chromatic Aberration. Lens profile correction helps, as does additionally applying negative adjustments to purple and aqua.

    I've been trying to edit the 'best' resulting image in PS, on a small bird silhouetted against the sky, using the healing brush set at just 2 px, but without much success - it's tending to make the bird itself less distinct, without concealing the light edge.

    Is there an easier way to do this. At the moment I've given up and just 'healed' the bird out of an image I'm working on..! It IS very small, so probably unimportant in this case but I would like to know how to sort this out.

    Many thanks (in anticipation!)
    Ian

  2. #2

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    Remco

    Re: CAs in BW conversion

    Perhaps show us some images illustrating the problem? (ideally original, edited colour image and B&W conversion)

    What I don't understand is how you can use the B&W mix sliders after conversion to B&W...

    Remco

  3. #3

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    Ian

    Re: CAs in BW conversion

    Quote Originally Posted by revi View Post
    What I don't understand is how you can use the B&W mix sliders after conversion to B&W...
    Remco
    I convert to BW just using the 'V' shortcut
    Adjusting the B&W mix is in LR Develop module, between tone curve and split toning

    Ok, here are some 100% crops:

    1. Original
    CAs in BW conversion

    2. Unmodified BW conversion
    CAs in BW conversion

    3. BW mix, blue dropped from +16 to -60 (a bit excessive! - to illustrate the effect)
    CAs in BW conversion

    4. Lens profile correction applied
    CAs in BW conversion

    5. BW mix: Purple and Aqua channels dropped to -100
    CAs in BW conversion

    We are looking at a band of just a few pixels, but it does stand out sometimes

    Ian

  4. #4

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    Re: CAs in BW conversion

    That is a common problem when shooting into the light, Ian.

    Adjusting the B&W colour sliders is much the same thing as adding extra saturation to the original, just a bit more precise.

    And increasing the blue saturation when you already have a 'blue fringe' problem just makes things worse.

    You could try working on the original and correcting with the Healing Brush set to Content Aware. Alternatively try a Clone Tool with a harder brush. Zoom in close and see if anything helps.

    Maybe adjusting the overall contrast of the original before doing anything else?

    The adjustments which you have tried have also considerably increased the sky 'noise' to what I would consider to be an unacceptable level.

    Yet another option might be to carefully select and extract the bird, after cleaning it up a little, then transferring it to a better sky. Possibly the best option in the long run? It probably depends on how well you can place a selection around the 'fringing' problem.

  5. #5

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    Ian

    Re: CAs in BW conversion

    Hi Geoff,

    Thanks for this...

    I'm just using this as an example to understand what's going on... and for tips on sorting it out... (I knew the blue adjustment - actually desaturating the blue - was extreme! Also completelty desaturating both Purple and Aqua to -100 )

    I tried the healing brush, but to prevent accidentally 'healing' the bird (!) it means working on just a few pixels at a time... which would be okay in this case, but when it's many tree branches I think that would be impracticable.

    Maybe this method of increasing contrast in the sky just has this drawback when there are dark objects set against the blue background, so best avoided?

    Ian

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