White Card or Gray Card ??????

Discussion in 'Digital Photography' started by David Griffin, Jan 5, 2004.

  1. This may have been asked and answered, but it seems to come up in here quite
    often.

    Isn't a Gray card used to calibrate EXPOSURE and a White card used to
    calibrate WHITE BALANCE? It seems as though some people think that the two
    different cards are "interchangable!" I tend to think; not.
    The White Card has only recently become an issue for still photographers
    since the invent of the digital camera. I have been using a Gray Card for 30
    years.






    http://www.sportsshooter.com/members.html?id=1990
     
    David Griffin, Jan 5, 2004
    #1
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  2. David Griffin

    Tom Thackrey Guest

    White Balance (which is actually color balance) can be set with any color
    neutral card, white or gray. It is possible that a white card will be
    outside the latitude of the sensor and consequently give a false value,
    however, it's hard to imagine proper use of the card that have this result.

    Setting exposure, this way, requires a known reflectance, 18% gray is the
    standard. You could use any known reflectance, 18% gray is convenient since
    it doesn't require adjusting the results.

    I use a gray card for both exposure and color balance. It works for film and
    digital. I have used a gray card for color balance when printing color
    negative film for many years.
     
    Tom Thackrey, Jan 5, 2004
    #2
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  3. David Griffin

    Don Coon Guest

    They are. You haven't been using any card for custom white balancing for 30
    years so that experience is irrelevant. Within reasonable limits, the
    camera doesn't care what shade of gray the "card" is as long as it's color
    neutral. As long as the RGB components are equal, 128,128,128 works as
    well as 255,255,255. Have you actually tried different shades of gray? I
    have.
     
    Don Coon, Jan 5, 2004
    #3
  4. They're more interchangeable than you think. You can read exposure from
    a white card just as well as a grey one. The only difference is you
    need to allow 2.5 stops more exposure if your reading came from a white
    card. I used this 30 years ago when shooting in snow (no grey in sight,
    just white) or when shooting in dim light (too dim for the meter to read
    a grey card, but white was bright enough).

    They also both work for adjusting white balance, as long as the exposure
    is set so that none of the 3 colours is clipped. All the camera needs
    is a sample of a colour-neutral non-black target to balance on.

    The one place I can think of where a grey card is more useful is as
    something to place in a test frame of a scene so you can use it to read
    exposure and colour balance for darkroom printing. A grey card ends up
    in the middle of the response curve of the negative, so it's probably
    better than a white card for this (because you probably want neutral
    midtones more than neutral highlights, if you can't have both).

    Dave
     
    Dave Martindale, Jan 5, 2004
    #4
  5. David Griffin

    George Kerby Guest

    Like I said before: Use the card.

    One side white=digital white balance for color balance

    Other side 18% grey=digital AND film for exposure

    It is versatile!
     
    George Kerby, Jan 5, 2004
    #5
  6. David Griffin

    Don Coon Guest

    If you are simply using the card to setup a Custom White Balance, you don't
    need 2.5 stops more when taking subsequent shots. You may need them to take
    the calibration shot but that's all. All the camera cares about are the
    relative RGB values of the calibration shot.

    Or did I misunderstand your post?
     
    Don Coon, Jan 5, 2004
    #6
  7. David Griffin

    Mike Engles Guest


    Hello

    If you have a histogram, use it to set the white point to just below
    clipping. That way you should maximise the S/N ratio of the camera.
    You can always make adjustments for the black end in software.


    Mike Engles
     
    Mike Engles, Jan 5, 2004
    #7
  8. David Griffin

    Tom Thackrey Guest

    You're right, good idea.

    (Please note that white point has to do with exposure not white balance.)
     
    Tom Thackrey, Jan 5, 2004
    #8
  9. That paragraph is talking about using a white card to set exposure
    instead of a grey card. Setting white balance was discussed in another
    paragraph. The overall point was that you can use either grey or white
    for either exposure or white balance, with little advantage of one over
    the other.

    Dave
     
    Dave Martindale, Jan 5, 2004
    #9
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