 BetterPhoto Member |
Gray Card for Exposure When and why do you use a grey card?
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anonymous A. |
Grey cards provide a neutral target for exposure meters, including the one in your camera. You use them to calibrate your camera against a handheld meter, to take a reading in difficult situations, to check the incident light reading against the reflected light reading of your camera's inbult system. They are especially useful with backlit subjects or when there is a bright light source like the sun or a light fitting in front of you. Also when the scene is dark on dark like a black cat on a dark blanket or light on light like a snowy landscape.
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Bob Chance |
If we convert our world to B/W, everything would be made of black, white and various shades of gray. Most things, especially grass and foliage, have a reflectance value of 18%. That is, they reflect 18% of whatever light is hitting them. Meters are designed to give an acceptible exposure to reduce the subject to this same reflectance value. It would be impossible for a meter to know exactly what the reflectance value should be for every subject. Some things reflect less light, some things more light. But in the broadest sense, 18% covers the majority. It's therefore up to the user to be able to distinguish those subjects that don't meet the criteria of 18%. If you meter a white wall, the meter still thinks it's a medium gray and the resulting exposure will give you a murky gray picture. But you know the wall reflects a lot more light than a gray card. More light means more exposure. The meter is giving less exposure to make it gray, so we must compensate for this and give it more exposure to brighten it up. The reverse is true for a black or dark wall. However, if you meter off of your gray card in the same light as your subject, then your exposure should be good always. But it's not always practical to carry an 8x10 piece of cardboard around. It's easier to learn how to read a scene and determine how the lighting would fool the meter and know when and how much to compensate. Incident meters read the light directly, not reflected light, but the exposure they give is to the same effect - that is, to expose an 18% reflectance to come out as 18% in your picture. You don't have to carry a card around with you as they read the light direclty. However, you can't always be in the same light as your subject. And for any meter to be valuable, you have to be able to read the light that is falling on your subject. Not much good to take a reading in the sunlight, if your subject is standing thirty feet away in the shade. The results would be disappointing, to say the least.
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