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Thread: GreTag MacBeth Color Checker - Lab Values

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    Member jerryreed's Avatar
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    GreTag MacBeth Color Checker - Lab Values

    I am a giclée maker, and my primary responsibility to the artist is to match the color of a known art piece. To make my business profitable, it is helpful to move as quickly to an exact color match as possible. In pursuit of this end, I have been investigating the utility of a tool that I current utilize, but imagine could be used still further with the effect of speeding up my workflow.

    The GreTag MacBeth Color Checker, publishes for each of the squares that make up the GT CC luminance and color value stated in the LAB Mode.

    Since it is part of my practice to utilize the CC to set the exposure and to neutralize the possibility of color casts, utilizing the GT CC, the card is included in every job as a reference.

    I am looking to learn if anyone has worked out a way to use COLOR BALANCE or CURVES adjustment layers to match the Lab values to those found in a photo of the GT CC. If I were able to develop a workflow that would consistently provide very close matching, it seems that would speed up my workflow.

    If anyone wishes to offer color matching suggestions that are not directly in response to the question that I have posed please feel free to jump in.

    Thanks in advance,

    Jerry Reed

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    Sr. Administrator Jack's Avatar
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    Re: GreTag MacBeth Color Checker - Lab Values

    Hi Jerry:

    I think for what you describe, you are best served with a dedicated capture profile. Of course the GMB Color Checker is used for that. There is no easy single click color balance that is going to balance all colors on the GMBCC accurately -- about the best you can hope for is relatively neutral grays, but it is unlikely the other colors will fall into place. Ditto curves -- a profile does generate a curve of sorts via its LUT, but that is not an intuitive thing to build by hand, usually requiring a spectro head to read the values and software to make the adjustment for the profile.

    In lieu of a dedicated capture profile, you could use your raw editor to make dedicated sets of adjustments for each capture, but this would be time intensive.

    My best advice would be to standardize your capture routine, especially lighting, then build a profile for a specific camera, lens and aperture setting in that lighting. This set of adjustments should remain accurate for various captures. (This is in effect is what is done with the Betterlight scanning back when copying artwork.)

    Cheers,
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Re: GreTag MacBeth Color Checker - Lab Values

    Its a bit unclear to me what your workflow is, but two suggestions:

    1. If you are talking about using Photoshop/ACR, what you are describing is essentially a calibration process - you should perhaps look at the Fors calibration script, or its various clones - they generate calibration values from a GM chart

    2. If you want an alternative to Photoshop, look at Aperture - it reads out directly in LAB, and has very easy adjustment when working in the LAB space

    Sandy

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    Re: GreTag MacBeth Color Checker - Lab Values

    Gentlemen,

    Thank you for your responses. I noted that ADOBE Labs has a profile maker for DNG files however, for a reason I cannot figure, the standard illuminant is 6,500 degrees Kelvin.

    So, looks like a useful tool, but based upon a light spectrum that would, in my experience be difficult to reproduce consistently. My Broncolor strobes are wonderfully consistent, but at 5,000 K.

    Jerry

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    Re: GreTag MacBeth Color Checker - Lab Values

    Quote Originally Posted by jerryreed View Post
    Gentlemen,
    My Broncolor strobes are wonderfully consistent, but at 5,000 K.
    Back to my Betterlight days, what mattered most with lighting was not so much net temperature, but rather that is was consistent color temp and full spectrum. As long as they are are relatively full spectrum (strobes should be) then you don't get destructive metamerism from odd colors found in various art media...

    Again, I would think that if your lighting is constant, your best bet is to build a profile for a given strobe setting/distance, and given lens at given aperture. This will be more accurate than any DNG or ACR/LR adjustment set. (You build the profile off a standardized set of raw conversion settings and standard profile output like Prophoto RGB. Then you ALWAYS capture with the same back settings, convert raws using those same settings, but "assign" your camera profile after that conversion in your image editor -- this is a bit counter-intuitive for many first-time users, so make sure you understand the proper procedure before embarking on this course.) Most modern lenses from the within the same manufacturer are now very carefully color and response matched, so the profile would likely be good enough for most applications. However, the pickiest clients will probably necessitate a dedicated profile for each lens. Note also your back can drift with age, so a profile is something you probably want to to rebuild periodically.

    You'll need a good spectro head and software combined with near perfect technique to build those camera profiles. Note that the heads and software vary: FWIW Gretag seems to generate the most accurate (neutral) grays, while Monaco generates smaller delta-e (more accurate color) on the color patch's. I opted for the most neutral grays... (The guys at Betterlight use both to generate three profiles -- a gray, a color and an averaged.) Note also that MacBeth cards change with age (fade) and become unreliable, and should be stored in a dark, cool environment. Note also older versions had significant variance between runs, so do some research first and make sure you get one from one of the more recent releases -- and for sure do not purchase/rely on a used one!

    Cheers,
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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