Kodack grey card values

Discussion in 'Photoshop' started by a, Feb 28, 2004.

  1. a

    a Guest

    If I take a photograph of the 18% Reflectance side of a Kodak grey
    card and open the image in Photoshop. What values should I expect for
    RGB in the Info palette?
     
    a, Feb 28, 2004
    #1
    1. Advertisements

  2. 18% reflectance is 18% L in the LAB mode. If you fill in 18,0,0 in LAB,
    you'll get RGB 48,48,48.
     
    Johan W. Elzenga, Feb 29, 2004
    #2
    1. Advertisements

  3. Sorry, it's too early in the morning, so I wasn't thinking clearly. It's
    50 L in Lab mode, so it's (depending on the RGB color space) RGB
    118,118,118.
     
    Johan W. Elzenga, Feb 29, 2004
    #3
  4. (Johan W. Elzenga) wrote in

    Well, that seems a bit more on the mark ;-)

    Curious, though. It would seem that hitting the midpoint in RGB would
    accomplish the same thing, except the midpoint (127,127,127 or thereabouts)
    appears to be about 1/3 to 1/2 stop brighter (photographic terms). Any idea
    why this is? Why would the midpoint in LAB not match the midpoint in RGB?


    - Al.
     
    Al Denelsbeck, Feb 29, 2004
    #4
  5. a

    Greg Guest

    128,128,128 is closer to the midpoint than that, isn't it? Punching the
    numbers
    into the CIE Color Calculator:
    http://www.brucelindbloom.com/ColorCalculator.html
    the luminance changes from 18.4 (L*=50) to 21.6 for sRGB 128,128,128.
    21.6/18.4
    is 1.17, which represents only 0.17 of a stop, which is not even 1/5th.

    Greg.
     
    Greg, Feb 29, 2004
    #5

  6. Well, two things to consider...

    1) The amount I stated was only a guesstimate, mostly from staring at
    photos onscreen too much of the day. It's also subject to my gamma ;-)...

    2) There's a difference between the luminance change of the actual
    monitor output, as measured by a light meter, and the onscreen
    representation of a photograph converted to 24-bit RGB. The monitor has an
    exceptionally narrow light range, and right at the moment, my pure black
    screen with no room lights is pegging about 5 seconds at f2.8, ISO 100,
    which is much higher than full moonlight on a grey card.

    And even then, it's highly subjective. Not only do the monitor specs
    come into play, but the exposure range of the film that is scanned. Slide
    films I use are 4-6 stops in latitude, print films about 9. And both of
    these appear "full gamut" in RGB space. So there's a teensy bit of play ;-)

    Bruce's site is producing some weird results for me, and I'm not sure
    what he's working with yet...


    - Al.
     
    Al Denelsbeck, Feb 29, 2004
    #6
  7. a

    a Guest

    My apologies for the crossposting, thats the first (and last) time
    I'll do that.

    And, if anyone else needs a reason NOT to crosspost - It's because you
    end up with intelligent, knowledgeable people NOT being able to engage
    in a structured conversation and NOT being able to develop their
    arguments with each other.

    The idea behind the original question was this:

    If I expose a roll of film and load the images into Photoshop with one
    exposure a picture of an 18% reflectance grey card, what should the
    RGB values be in the Info pallete? The consensus seems to be approx
    R=119, G=119 and B=119. If I then correct any discrepancies - can I
    apply the same correction to the rest of the roll of film?
     
    a, Feb 29, 2004
    #7
  8. In one word: yes. Providing that all the shots were taken under the same
    circumstances, of course. For example shooting in a studio. And
    providing that all the images were exposed the same, meaning exposed
    with manual settings. If the camera is set to automatic exposure, you
    can NOT assume it will always need exactly the same correction.
     
    Johan W. Elzenga, Feb 29, 2004
    #8
  9. a

    Greg Guest

    The f-stop change I gave for the monitor would be accurate for a monitor
    which
    outputs 100 Cd/m^2 for it's pure white. My monitor output's 75Cd/m^2 when
    calibrated for 5000K, so the f-stop change for my monitor when calibrated
    for 5000K would be even *less* than the value I calculated. At 6500K, the
    output
    of my monitor increases to 88Cd/m^2, which is still less than 100, so again,
    the
    f-stop change would still be less. Those of us with LCDs, which are
    typically
    brighter than CRTs, may experience a greater f-stop change than the value I
    calculated.
    (LCDs can typically go over 100Cd/m^2)
    It may help if you ensure that you select "scale XYZ", "scale Y", and "scale
    RGB".

    Greg.
     
    Greg, Feb 29, 2004
    #9
  10. a

    Greg Guest

    I don't think I did this calculation properly.
    I think the proper formula to calculate the relative f-stop change from one
    luminance to another is:

    f-stop(relative) = (ln(Y2) - ln(Y1))/ln(2) =
    (ln(Y2) - ln(Y1)/0.693

    Where Y is Luminance.

    Using this formula the result is 0.23 of a stop.

    I'm still not 100% sure of this new formula though. (can anyone confirm?)

    Greg.
     
    Greg, Mar 3, 2004
    #10
  11. a

    Greg Guest

    f-stop(relative) = (ln(Y2) - ln(Y1))/ln(2) =
    oops - left out a bracket:

    f-stop(relative) = (ln(Y2) - ln(Y1))/0.693

    Greg.
     
    Greg, Mar 3, 2004
    #11
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.