A card with a neutral gray surface that usually reflects 18% of the light that falls on it. Light meters are calibrated to make a scene look middle gray, so a reading from a gray card gives a reliable exposure. You place the card in the same light as the subject and take a meter reading from it. This helps avoid overexposure and underexposure, especially with very bright or very dark subjects. A gray card can also be used to set a custom white balance.
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