A&P Gray Card

Discussion in 'Lifestyle' started by CRC, Dec 3, 2004.

  1. CRC

    CRC New Member

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    I know it's used in setting white balance and exposure, but how and why exactly? On my point and shoot digital cameras, I've always set the custom white balance by finding something white and using that as a reference, but I guess that's wrong? :hs:
     
  2. hash browns

    hash browns lolcathlon champion OT Supporter

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    The meters in the cameras are built to measure based on something that's 18% gray. Some say 12% gray. Values in this range are considered "neutral gray." Because the meters are calibrated this way, having an 18% gray card will give 'correct' exposure values when metered off of.
     
  3. Jcolman

    Jcolman OT Supporter

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    A little primer on light meters and their use.

    There are two types of light meters, Incident and reflective. An incident meter reads the light that falls on a scene whereby a reflective meter reads the light reflecting off objects. All video cameras and still cameras that have a built in meter use a reflective meter. The problem with a reflective meter is when the camera "reads" a very light or very dark object it will give you an incorrect exposure. Most cameras "average" the light reflecting from a scene to give you an exposure valule.

    For the most accurate metering of a scene with a reflective type light meter, professionals use an 18% grey card and measure the light that is refected off this card.

    A grey card is placed in the scene and the camera is zoomed in so that the grey card fills the frame. The exposure is then read and set. In the absense of a grey card you can use the palm of your hand as it is nearly identical to the value of an 18% grey card.

    A grey card should not be used for white balance. It is used only for determining a correct exposure. A white card or other white object should be used for white balance.

    Some video professionals use a "chip chart" in place of a grey card. A chip chart contains a scale from pure white to pure black and either seven or eleven shades of grey in between. A chip chart can also be used for color correction in the post process when shooting film.
     
  4. mojito

    mojito New Member

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    In addition to the above mentioned uses for a gray card, tey're also used with film to determine densities to find out the proper exposure index and guide numbers of flashes to see the true working iso of a film compared to the manufacturers stated value and the actual flash output as opposed to the manufacturers stated value

    The gray card needs to be lit properly for any of the above uses to work correctly.
     
  5. MrBucket

    MrBucket OT Supporter

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    For using a custom white balance for digital I sometimes take a coffe filter an put it over the lens and aim it in the direction im shooting. Works really well.
     
  6. CRC

    CRC New Member

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    So it's used for exposure and NOT white balance? That makes more sense now :o
     
  7. Jcolman

    Jcolman OT Supporter

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    Exactly. You also know that you can "warm" a scene by white balancing your camera on a light blue color and "cool" a scene by balancing on a pale yellow color.
     
  8. hash browns

    hash browns lolcathlon champion OT Supporter

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    Some grey cards have 18% grey on one side and are white on the other.

    Thus they can be used for exposure and white balance.

    Maybe this is what got you confused.
     

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