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rgb cmyk luminosity shifts

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  • rgb cmyk luminosity shifts

    hi all
    hope you can help me with this.
    I work in the publishing industry, photo-retouching images for print.
    I use an rgb workflow for all my image work, and then convert to cmyk to place an image in InDesign. The more I learn about working with photographs, the more I seem to be having this issue:
    Printers and cmyk-workflow retouchers recommend that there be no holes (areas of missing information) in the cmyk separations in the final placed cmyk image file, in order to minimise banding (especially not when using gravure printing (which clips the possible image information as it is able only to reproduce tones of 7% or more)).
    In other words there must be an even tone in all channels, including cyan (especially on skin-tones). But the techniques I am using more and more (pwl, dodge and burn) leave me with a glowy, glossy image in rgb, and an image full of holes (on the highlight areas) in cmyk.
    When I fill in these holes (using a combination of apply image and information copying from other channels), the glowy, glossy skin effect is minimised.
    How do I get around this?
    What am I doing wrong?
    please help.

  • #2
    Re: rgb cmyk luminosity shifts

    It might help to use your proof setup when retouching. Got to view-proof setup-custom and choose your eventual cmyk profile. Now you can at least see what you will get when you eventually convert. For me, I never use dodge and burn for print photos. I find you loose too much detail. I would use masks or, even easier, use the history brush to paint back in detail. Hope that helps.


    • #3
      Re: rgb cmyk luminosity shifts

      Okay - so, you must become one with the info pallet !

      Mask that area, apply a channel mixer adjustment layer, and adjust the "percentage" of each color channel.

      Place the eyedropper over the area and use your info palet to see the color percentages go down until you hit 7% (total).

      On your info pallet, you can change the options to show a "total ink amount"

      You use your INFO pallet to guide you on how much to decrease, per channel.

      watch the numbers ("ink percentage") in the info pallet as you decrease the colors, if it's CMYK, you can have a breakdown like c2,m2,y2,k1 which will give you a total density of 7, maybe you only want 7% black and no other color.

      All depends on your design!

      if your main objective is to decrease enough ink to leave you with no more than 7% total ink density in that area - i know of no other way to do this EXCEPT by retouching carefully.

      I was director of a prepress division where we prepared images for both Gravure and heatset offset - this is indeed a real issue that can't be ignored (as I am sure you have learned)

      Good luck and watch them numbers - it is like watching your gas gauge - ignore it and you get in trouble!

      hope this helps
      Michael Jahn - Slightly used PDF Evangelist
      Simi Valley California


      • #4
        Re: rgb cmyk luminosity shifts

        Here's a little trick I use, when I know my targets.

        I used to retouch computer cases, for a catalog company. We found that a color, let's say 3C2Y2M, was good for the color of the computer cases.

        Rather than go in and adjust each channel individually, I'd go to my levels palette. if you double click on your eyedroppers, in this case, on the white point, it pulls up your color picker. Here you can change your white point.

        So in my case, I'd change the CMYK values to 3C2Y2M0K, and OK it. then when I clicked, with the eyedropper, on my white point... POOF! My whites become a 3/2/2 dot.

        These days, you can do this with an adjustment layer, but back then I had to hike two miles through the snow, to the layers free Photoshop v2.5...

        But be careful converting images from RGB to CMYK, with adjustment layers, though. Freaky things can happen to your color corrections, if you let Photoshop flatten it on the fly to CMYK.

        Hope this helps!



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