What is a white card used for?

Discussion in 'Exposure Discussion' started by marcmarc, Mar 28, 2010.

  1. marcmarc

    marcmarc Member

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    Many moons ago my father gave me a grey card that was plain white on the reverse. He told me it had something to do with exposure but I couldn't recall what he said. I also read a book once where it mentioned that the white side of a grey card could be used for very low light/night shooting. I recently did some street photography at night using Delta 3200 and my Sekonic incident meter. I haven't been able to print anything, but the negs look OK; what I would expect from this film rated at 3200. I usually try to expose it at 1600, but I needed that extra stop of shutter speed. My Sekonic has a spot meter so if the white card is for such shooting conditions, is there any advantage for me to simply spot meter it for a general exposure reading, or should I just continue with incident readings? Thanks for replies.
    Regards,
    Marc
     
  2. marcmarc

    marcmarc Member

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    By the way, I understand that metering a grey card in normal light will give me the same reading as an incident. So it would seem to me that metering a white card at night will underexpose since white reflects more light.
     
  3. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member

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    The white side of the card is useful when there isn't enough light to get a reliable reading from the grey side, but that doesn't mean you should use the reading from the white side directly, or whites in the image will appear grey. For negative film, if I were metering in this way, I'd place the white side around two to three stops over the grey side in normal lighting conditions (open up 2-3 stops from the reflected reading of the white side). For slide film, I'd place the white side 1.5 stops over the grey side (open up 1.5 stops from the meter reading on the white side).

    Now, would I actually meter this way? Not likely, at least with negative film. With negative film, it's better to meter for the shadows and adjust development for the highlights in the image. With slide film it makes more sense to meter for the highlights. In either case you are metering for what you must get on film. If you underexpose negative film, you'll have no shadows, but if you overexpose moderately, you may have a difficult negative to print, but you'll probably have some highlight detail. If you overexpose slide film, you lose the highlights, but if you underexpose slightly, you've got a bit more latitude before blocking up the shadows.
     
  4. markbarendt

    markbarendt Subscriber

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    White or grey doesn't matter, it's simply a reference point.

    The reference point can be used to set camera exposure by "placing" the chosen reference point at a specific point on the film curve.

    If you actually take a reference photo of the card you can also use that as a reference point when printing.
     
  5. Kevin Kehler

    Kevin Kehler Member

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    I use a grey and white card for reference, as in knowing that this is middle grey, how many stops different is my shadow? Knowing that this is white, is it different than what I am metering and if so, what kind of development compensation am I going to use? Thus, I do not use it to meter per se, but rather I use it to give references to my "pre-visualization" in order to ensure my calculations are correct.
     
  6. Terrence Brennan

    Terrence Brennan Member

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    When you use the white side of the card, which reflects about 90% of the light falling on it, as opposed to 18% for the grey side, you should divide the film speed set into your meter by a factor of 5 (90/18 = 5), or open up your lens 2-1/3 f/stops.
     
  7. wclark5179

    wclark5179 Member

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    Color and color balance is a complicated topic. Several things. Exposure, film or the other method, viewing it to correct it be it an enlarge, computer screen and how it is presented to the viewer, print, electronic. In print lots of things to consider, where it will be viewed, the framing, the wall color(s), type of lighting, paper it is printed on just to mention a few.

    I find there is a difference just looking at identical prints but with different finish, one is, for example, gloyy while the other is matte. Or viewed with tungsten lighting, trac lighting with halogen bulbs, fluorescent or natural lighting at different times of the day.

    There is a lot more to it than this but I've gotta run cause I'm teaching a class today to a bunch of photographers.

    Here are a couple of links to get you started:

    http://en.wikipedia.org/wiki/Color_balance

    Ed Pierce has a device to take a peek at:

    http://www.photovisionvideo.com/store/shop.mvc?Screen=CTGY&Category_Code=DCT

    Adobe has some interesting stuff as well.

    I used to print color in my darkroom years ago but gave it up as I could delegate it out to a lab.
     
  8. marcmarc

    marcmarc Member

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    Thanks for the replies and links. So I'm probably safe just using incident readings at night (as long as I'm in the same light of course which can be tricky).
     
  9. Q.G.

    Q.G. Inactive

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    Unless, of course, it is too dark for incident light, but still just about light enough for a reflected meter reading taken off a white card. :wink:
     
  10. Galah

    Galah Member

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    What I understand you are meant to do is: Take a (reflected light) reading off your white card and then multiply your exposure by five.

    The most accurate way to do this is to multiply your shutter time by five: ie, if the reading suggests f/2 @ 1/8th, you should give it f/2 @ 5 x 1/8th, or f/2 @ 5/8ths (the nearest being 1/2 sec?).
     
  11. markbarendt

    markbarendt Subscriber

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    This is a generic of a rule of thumb for placing white on the curve about 2 stops up from middle gray.

    This may be a very accurate guesstimate for Galah's purposes but the accuracy is dependent on the film in use, the scene, the subject, the style, and the intent.

    Example; I did an event at a ski resort, the start of a race course, a few weeks back the participant's faces were in open shade, the snow around them was in full sun, there was a dark evergreen forest behind them.

    The faces of the people were in a low contrast setting, the rest of the scene is high contrast.

    In a situation like this you have to decide what is most important to you and which method/tool for metering will work best.

    A white card two stops up may have led me astray in that situation.

    There is no substitute for experimenting for yourself.
     
  12. Q.G.

    Q.G. Inactive

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    How much to correct when using the white face of the card instead of the grey is easily determined by measuring both to see the difference.
    If you use that compensation when using the white side, you will always get the exact same result you would have gotten using the other, grey side.