18% Gray Card
An 18 Percent Gray Card is a simple gray-coloured card which uniformly reflects 18% of the light which falls upon it. Gray cards can be used as a reference to set the camera exposure or to calibrate a light meter (light meters are used to determine which exposure setting is required to produce a medium gray tone).
Gray cards are usually made from coated cardboard or polystyrene and come with a protective cover. They are an inexpensive and useful addition to the photographers' kit.
How to Use a Gray Card
To set your exposure with a gray card, first make sure the camera is in manual mode or is able to hold its settings when you half-depress the shutter button.
Position the card immediately in front of the subject, ensuring that the lighting on the card is exactly the same as the lighting on the subject. If you are using a light meter, take your reading now. Otherwise, half-depress the camera shutter button to save the exposure setting until you fully depress the button (and take the picture).
(Note: This process is very similar to performing a white balance).
- At the start of your roll of film, take one photo of a gray card. This acts as a colour correction reference for the film processor.
- Do not use any old gray piece of cardboard for a gray card. You really need a professionally-made one.
- When you first purchase a gray card, run a series of tests to establish exactly how it performs in both bright outside light and lower artificial light. Use slide film rather than print, as slide film does not correct exposure errors.