The Old Gray Card Ain't What It Used To Be! - Brief Article

Musings and Myths About an Expensive Piece of Cardboard

The following is an interesting paragraph, taken from the "The National Geographic Photography Field Guide" (by Peter K. Burian and Robert Caputo, National Geographic Society, Washington DC, 1999): "Scientific studies now indicate that an average scene actually reflects 13 percent (not 18 percent) of the light that falls on it. For the sake of consistency, gray cards have continued to be 18 percent gray, as is the one in this book. When using any 18 percent gray card for substitute metering, increase exposure by a half stop (+0.5 compensation factor) for most subjects, as Kodak recommends."

This 13% reflectivity of the average outdoor scene was established by Kodak research almost 60 years ago and the recommendation alluded to by the Field Guide is found in the instructions that come with a Kodak gray card, which reads in part: "Meter readings of the gray card should be adjusted as follows. For subjects of normal reflectance, increase the indicated exposure by 1/2 stop."

Mathematically, this works out to setting the reflectance at 13%, rather than 18%. (Note: When the instruction sheet for the Kodak gray card was revised back in the '70s, someone inadvertently left off this part about compensating. Now it's back.)

This has led to a great deal of controversy in the photographic community as to whether camera and other exposure meters are calibrated to an 18% reflectance standard or to some other, the most often mentioned being 10 to 13%. Those who espouse the latter often cite meter technical data from manufacturers such as Sekonic, where the reflectivity constant is set to 12.5 and Minolta, where it is set to 14. This, however, is a misinterpretation of the factors involved.

With light meters that record both incident as well as reflected light readings, it is necessary to be able to correlate the reflected light readings with the incident light ones. For this reason, meter manufacturers use reflectivity constants to calculate illumination from a standard luminance value. The constant chosen is a function of the manufacturer's design and choice. At a value of 12.5, for example, the interpretation of this constant is that a reflected light reading taken from a source of known luminance is 12.5% that of the incident reading of the illumination of the same object. The bottom line is that the meter's reflectivity constant and scene reflectance, the source of the confusion, are not the same. The reflectivity constant is not related in any way to the 18% reflectance standard.

Some models of cameras, particularly point-and-shoot and entry-level 35mm cameras, do have light meters built into them which the manufacturer has adjusted to give more exposure than would be indicated by a hand-held meter. Since underexposure is the most common cause of dissatisfaction of owners of these cameras, manufacturers increased the exposure to solve the complaints. This, however, is certainly not done with handheld meters and cameras intended for other than entry-level photographers. The Minolta IV users manual, for example, states: "Standard reference subjects may have a reflection factor somewhere between 12% and 26%. Minolta bases its readings on a reference subject with an 18% reflection factor." The Sekonic Studio Deluxe L-398 manual also states that it is designed around a standard scene reflectivity of 18%. (Note, once again, these are not reflectivity constants.)

Choosing a scene reflectance value provides a source of known luminance for the meter to read. Although the standard is 18%, it could be 10%, 13%, or even 90%. It doesn't matter as long as the photographer knows approximately where on the film's exposure latitude the standard falls and how to adjust from that position to the approximate midpoint position. Thus, many photographers re-rate their film speeds, e.g., Fuji Sensia from 100 to 80, and Velvia from 50 to 40, which is equivalent to increasing exposure by 1/3rd of a stop and fairly close to the Kodak recommendation.

Although 18% gray cards were designed to meter for graphic arts copy, where they represent the midpoint for a black and white image with no shadows, this standard gives the photographer an equal number of brightness steps that show detail on each side of the film's midpoint.

When you use a gray card (or any incident light meter), the resulting camera settings are not dependent on the distance from the subject because subject brightness depends on light source-to-subject distance, not camera- or meter-to-subject distance. However, there are correct ways to use all metering schemes and the gray card is no exception. Try this. Have a friend hold up a gray card and, with your camera on a tripod, spot-meter the card. You will find the meter bouncing all over the place, usually over a 1-stop range. If you follow the set of instructions that come with the card, it will tell you how to angle it between the subject and the light source to avoid glare. Also, the gray card needs to be close enough to fill the metering pattern you are using. If you are using matrix mode, for example, make sure the card covers the entire frame.

It is important to understand that a gray card (or the "sunny 16 rule" or any exposure meter, for that matter) will not tell you how to get a proper exposure. It merely tells you how to make what it meters, middle-toned. All metering schemes are merely first steps; the decisions of the photographer are the final ones.

COPYRIGHT 2000 Photographic Society of America, Inc.
COPYRIGHT 2001 Gale Group