Still trying to figure the grey level of an 18% grey card (was: If I shoot a grey card, should this

Discussion in 'Digital Cameras' started by Alan F Cross, Feb 29, 2004.

  1. Alan F Cross

    Alan F Cross Guest

    I had several responses on this, all giving different answers!! I have
    also seen three definitions of the meaning of 18% grey:

    Definition 1: An analysis of 'average scenes' (whatever that may be)
    showed they were 18% reflective. Can't imagine there is such a thing as
    an average scene that would be in any way useful!

    Definition 2: the mid point of a 5-stop scale. I can see that 100/18 is
    around 2.5 stops. This sounds more plausible, but if you work on a
    6-stop range, the point moves to 12.5% so there's little justification
    for the figure.

    Definition 3: It's just an arbitrary figure to which most exposure
    meters are calibrated, by convention (and some do something else).

    Perhaps the truth is a combination of these. But there must be a value
    (between 0 and 255) that represents the same shade of grey as an 18%
    card. I know that there is the gamma question, but surely this is to
    essentially remove the non-linearity of the display tube, not to distort
    the grey-scale itself.

    A mid-grey that is scanned and then printed should look the same, no
    matter how screwed up it is on the display. I would have thought that
    the tube should *try* to display the same grey that is being scanned and
    being printed (within the constraints of reflective vs transmissive

    So, the question remains: if I create a grey of 127 or 128, *should* it
    look like an 18% grey card? If not, what level of grey should?

    The root of my question is the following problem:
    When I send mono work out for printing, I attach a grey scale of equal
    steps (0, 16, 32, 48,.....240, 255). It is easy to recognise 0 and 255
    when printed, but how do I tell them that their rendition of mid-grey is
    too light or too dark?

    I would need a full wedge to prove linearity across the range, and to
    ensure that there is no black or white clipping. But I really want a
    separate grey patch, labelled "this should look like and 18% grey card
    when printed". Perhaps this is 127/128, or perhaps it's something else
    (and that is the question!) The only reason to choose 18% grey is that
    they would certainly have a reference card available to compare.

    I'd appreciate any further comments on this perplexing (but essentially
    simple) issue.

    Alan F Cross, Feb 29, 2004
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  2. Alan F Cross

    Don Coon Guest

    Have you tried scanning a grey card?
    Don Coon, Feb 29, 2004
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  3. Alan F Cross

    Tony Spadaro Guest

    A grey card comes out at about RG&B 160 on a reasonably calibrated
    Tony Spadaro, Feb 29, 2004
  4. Alan F Cross

    mark_digital Guest

    A few years ago there was a discussion in this group along the same line you
    seem to be asking. I'm sure it made a few go out and buy a gray card and
    understand a bit better about nuetral gray and it's visual perception under
    varying light sources and intensities.
    If you printer and paper were capable of producing a sample that could
    reflect 18% gray then your problem would be solved.
    mark_digital, Feb 29, 2004
  5. Alan F Cross

    JPS Guest

    In message <>,
    That would be (0.18^(1/2.2))*255 = ~117. Most cameras and RAW
    converters target ~127, though.
    JPS, Feb 29, 2004
  6. Alan F Cross

    JPS Guest

    In message <>,
    I would think that the answer depends on the paper used. Paper is in
    its own little world of low contrast, and blacks that are grey in the
    real world. I don't think prints even attempt to center around a grey
    JPS, Feb 29, 2004
  7. Alan F Cross

    JPS Guest

    In message <PAd0c.38058$%>,
    Huh? Monitors don't have RGB values. They are passive analog devices,
    displaying voltages created by the gfx chips. An average exposure will
    be about 127 with most cameras and RAW converters. I've seen 117 as
    well (which is what 18% grey should be) in one context, (but I don't
    remember where).
    JPS, Feb 29, 2004
  8. Alan F Cross

    Tony Spadaro Guest

    Quite wrong. If you would take the time to compare a grey card to the greys
    available in Photoshop you will find - that if your monitor is fairly well
    calibrated you will get about 160. Don't talk theory when it takes vary
    little work to see that your are flat out wrong. If you don't have a grey
    card - buy one.
    Tony Spadaro, Feb 29, 2004
  9. Alan F Cross

    JPS Guest

    In message <kog0c.55216$>,
    You're not making one bit of sense. Inability to communicate and
    inability to detect failed communication and initiate a retry is an
    epidemic disease, and you are infected.

    Where is this "160" figure coming from?

    A grey card's level in a room depends totally on ambient lighting.
    JPS, Feb 29, 2004
  10. Alan F Cross

    Alan F Cross Guest

    It shouldn't have anything to do with the monitor!
    Alan F Cross, Feb 29, 2004
  11. Alan F Cross

    Alan F Cross Guest

    What has it got to do with the paper used? Any media variations should
    be taken care of in the printer profiling and the driver. You shouldn't
    be adjusting your image to each media used, you should be adjusting the
    printer transfer characteristic.
    Alan F Cross, Feb 29, 2004
  12. Completely correct.

    You cannot map a grey card to any level in your digital picture
    or on your monitor. A grey card simply reflects 18% of the light.
    A monitor does not reflect light - it emits light.

    Roland Karlsson, Feb 29, 2004
  13. Alan F Cross

    JPS Guest

    In message <[email protected]>,
    So what the hell was he talking about? I can see why people killfile
    him; he writes, but doesn't read.
    JPS, Feb 29, 2004
  14. Alan F Cross

    JPS Guest

    In message <>,
    People who don't really read and just look for keywords to agree with or
    pounce on generally get that communicative feeling, even if
    communication isn't happening.

    Anyway, any solid-shaded grey object comes out at 127 in photoshop with
    most cameras and software, not 160.
    Maybe a sicilian or a black Irish (my paternal grandmother was one of
    these, and my sister has a dark complexion), or someone with a South of
    France tan, but most caucasians are about a stop more reflective than
    18% grey.
    JPS, Feb 29, 2004
  15. Alan F Cross

    JPS Guest

    In message <>,
    Doifferent paper and ink combinations have different dynamic ranges,
    regardless of what the software tries to do.
    JPS, Feb 29, 2004
  16. Alan F Cross

    Don Stauffer Guest

    An 18% grey card has a reflectivity of 18%.

    Now, how it is used in exposure determination is something else. This
    is to a large degree up to judgement and desire of photographer when you
    use manual metering. All of your 'definitions' could apply.
    Don Stauffer, Feb 29, 2004
  17. Alan F Cross

    Tony Spadaro Guest

    You are a complete fool - and a stubborn, blind fool at that. Since you
    cannot learn I suggest you go live in my killfile with the rest of the
    idiots. Good bye loser.
    To others who may be curious: The "160" figure comes from comparing the
    colour of a grey card with the colours on a monitor and on prints made from
    swatches of those colours. Mr Sheehy has obviously never done a single test
    in this matter - and would rather make noise than know the truth. Ignore the
    Tony Spadaro, Feb 29, 2004
  18. Alan F Cross

    Tony Spadaro Guest

    If the monitor is calibrated -- even only moderately calibrated you will get
    approximately the same grey at 160 as the grey card -- isn't that clear? Try
    it. Open photoshop, make an 8x10 file and fill it with 160 grey (160R, 160G,
    160B) then hold up your grey card -- they should be close - very close. Now
    print the file -- your printer will probably add a cast to the file but it
    will still be close.
    Tony Spadaro, Feb 29, 2004
  19. []

    What would be a reasonable question might be:

    - if I expose for white = 255, what grey level should an 18% grey card be?

    Of course, the answer depends on what gamma correction the camera software
    has used when making the 8-bit image from the original sensor data.....

    David J Taylor, Feb 29, 2004
  20. True. A more subtle question is "what does white mean?". If you expose
    to map white to 255, does that mean the white of white paint, or a white
    tablecloth, or a white shirt? All are diffuse reflectors that reflect
    about 90% of the incident light, but they are likely to have somewhat
    different colour. Also, do you expose for your white reference in
    sunlight, or in shadow, if the scene contains both?

    Or do you expose with some headroom above flat white in order to be
    able to capture details from specular reflections, such as the sun
    reflecting from shiny metal? It's not practical to avoid all
    overexposure of such reflections, but it's often desirable to have them
    at least a bit brighter than matte white.

    The digital photography question "what pixel code should a grey card be"
    has the chemical photography equivalent "what negative/transparency
    density should a grey card be exposed and developed for. In both
    domains, there is no absolutely correct answer - the best exposures in
    different circumstances end up with somewhat different grey card
    exposures. There are only guidelines to get someone started in the
    right direction.

    Dave Martindale, Feb 29, 2004
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