To Use or Not to Use a Gray Card? That is the question…
In the era of film photography, when every single frame equated to an actual expense for the photographer, any tool which made the results of picture-taking more reliable was a valuable commodity. The gray card, popularly manufactured by Kodak, was/is commonly an 8.5 x 11 inch piece of card stock with an 18% gray (18% refers to the amount of reflectance) color painted on one or both sides. The gray card is significant because when light meters were invented, their designers observed that an average scene (almost anywhere in the world) yielded an 18% reflectance. In other words, most situations, from the scene outside your bedroom window to a bustling Manhattan street corner, possess a combination of lights and darks which, when considered together, absorb about 82% of the visible light spectrum hitting them. Therefore, when your camera or reflected light meter provides you with an exposure reading for the scene, it is assuming that what is accurate is a situation that represents that 18% reflectance value. Generally, this works just fine. However, in a composition that comprises either a lot of blacks or a lot of whites, this can become a problem.
Take, for instance, a snow-covered landscape. If your camera’s light-meter were left to its own devices, your resultant exposure might appear more ash-covered than snow. This is because your camera is assuming that the scene is reflecting 18% of the light, when it is more likely reflecting much more than that, due to the all of the white precip. A gray card can be used to rectify this sort of problem, and in situations more nuanced. To use the card effectively, the photographer simply places it in front of his/her subject, with the gray side facing the camera or light meter, and takes a reading. The photographer then uses this reading to photograph the scene. Note, the card must fill the frame (the gray card is a scene substitute), and must be in the same light as the subject (if a landscape, simply place the card in front of the camera; the sun is not going to change its value even if your subject is a mile away, but watch for clouds). Do not cast your shadow onto the card. By using a gray card in this way, you will get accurate blacks and whites in situations that do not conform to what your camera expects to see.
But in an age of digital cameras and 3-D Matrix Metering, is this extra step even necessary? Short answer is not really, unless, of course, you’ve got the time and don’t mind the extra step. In full disclosure, I very seldom use a gray card. Modern, in-camera metering is very good at accounting for things like snow, or tuxedos, etc. Not to mention, you can view the image right after taking it. Additionally, once you become familiar with your camera and how it records subjects, you become pretty good at knowing when to throw on some exposure compensation. Nonetheless, understanding how to use the gray card and why/when is fundamental to getting the types of images that you want on your first attempt.
An alternative to using the gray card in an outdoor situation (can be used indoors too, but it’s trickier) is to use the palm of your hand. Most skin types reflect between 20-35% of visible light (the darker your skin type, the less reflective it is), so you can expect to increase your exposure, using a palm reading (ha ha), from between 1/3 to 1 stop, depending upon the darkness or lightness of your skin.