BetterPhoto Q&A
Category: New Answers

getting correct exposure

To love this question, log in above
August 27, 2006

To love this comment, log in above
August 27, 2006

Don't read anything above "No need to hold the Gray Card" Just skip the first part.

The standard used to calibrate exposure meters and film speed is the “Gray Card”. This simple cardboard tool was carefully constructed. It has a surface that reflects away 18% of white light that falls (incident) on it. Stated another way; shine 100 units of light on a gray card, 18 light units will be reflected away and 82 units will be absorbed by the card. Math too complicated to broach here describes the Gray Card as surface having an optical reflective density of 0.75.

How could it be that this simple device server as a standard for photographic exposure? Historically, two photo engineers Hurter and Driffield spent most of their careers putting this together for us. They studied how films react to light and subsequent development. These pioneers established the methods used today for determining film speed and proper exposure for film and print, black & white and color and yes, even digital systems.

Their theory forms the basis what we call the Law of Reciprocity and the Ansel Adams zone system.
Briefly: The Gray Card shade matches the center shade the exposure range.
Include a Gray Card in the scene and if correctly exposed and developed, the image of the Gray Card on the film will have a transmission density of 0.75 + film base (i.e. same shade as the Gray Card).
If a print is made of this negative and the print was properly exposed and developed, the image of the Gray Card on the print paper will have a reflected density of 0.75 (i.e. the same shade as the Gray Card). Thus the only shade that exhibits the same shade in real life plus on the finished film and on the finished print is a target with a reflected density of 0.75 i.e. the 18% Gray Card.

Light meters, projection printer meters are calibrated using the 18% target as are most other devices used in photo instrumentation.

No need to hold the Gray Card:
The light meter (in this case the camera’s through-the-lens-meter) is brought close to the card, just as if you are making a close-up photograph of the card. The card can be held by another person or propped up somehow.
The light falling on the card is to illuminated with light that has the same brilliance and quality as the light falling on your subject. If the vista is daylight illuminated, one can assume the card and the subject are illuminated the same provided your camera and/or your body is not shadowing the card.
Take the reading than remove the card and expose using the exposure indicated by the meter.
The card’ image must have a large presence in the negative to be useful. Consider taking a second picture within minutes of the first, using the same exposure, this time placing the card quite close to the camera. Do not change anything, don’t refocus.

Without instrumentation, you can check your work. Using a hole punch, make a circular hole in middle of the image of the Gray Card on your print. Overlay your print on the actual Gray Card. The real Gray Card shade is now seen through the hole. Only if the exposure and processing is correct will the real Gray Card be a close match to you photograph of a Gray Card.

Best of luck

Alan Marcus
ammarcus@earhlink.net

To love this comment, log in above
August 27, 2006