The 90-Percent Rule—Part One—Metering

[rolandogomez]

What I like to call the 90-Percent Rule in photography is a fundamental that can work for a photographer in many manners when properly understood. Because it comes to play in various photographic aspects, this first article will focus on metering and future articles will cover other applications of this photographic principal.

First, let’s look at what the 90-Percent Rule is in photography. In a nutshell, it’s a fundamental, a principal, not just a rule, that simply stated applies to reflected and absorbed light. Whatever is pure black, will absorb 90-percent of the light that hits it, whatever is pure white will reflect 90-percent of the light that hits it. As simple as that seems, many photographers forget the other part of the rule, there is 10-percent light reflected from pure black and 10-percent of the light absorbed from pure white. The key word in all the states about the 90-Percent Rule is “pure.”

Not all black or whites are pure, though many are close. To use the 90-Percent Rule effectively, a photographer must learn to judge shades of gray, or all the tones and mid-tones between pure white and pure black. In photography, black and white and the shades in between are not colors, they are tones and some tones are more reflective than others. The darker the tone, the more light it absorbs and the less it reflects, the lighter the tone, the more light it reflects and the less it absorbs.

Now that we understand everything to the black side of the scale (think of your histogram) is absorbing and everything to the white side of the scale is reflecting, we can apply this when metering our subject when illuminated by artificial light. If we build a set, then meter it and the result is F/8 off our flash meters, that would be the actual aperture we’d use to properly expose the set, however, throw in a model/subject in the set and now you’ll have to consider her skin tone—what tone is it? Is it darker than middle gray? Is it lighter? How much darker or lighter?

Now many will argue about the accuracy of the meter based on 18-percent gray, a middle tone is all that is needed, but in reality, when you take the 90-Percent Rule into account, it’s only true for the scene, not the subject. The flash meter is measuring the light falling on the subject, incident light, not the reflected light the digital camera’s sensor will capture from the subject’s actual skin. Learning to judge how much light a model’s skin will reflect takes skill, but the easiest solution is to stop down 1/3 of an F/stop for lighter than average skin tones and to open up 1/3 of an F/stop for darker than average skin tones.

Think of your incidental flash meter as starting point based on 18-percent gray, similar to the meter in your digital and film cameras. The color of 18-percent gray is close to the gray street asphalt (not black top) found on most highways and parking lots. I’ve worked as a photographer/photojournalist in almost 40 countries, photographed tens of thousands of people and still have never seen anyone with 18-gray skin.

When properly understood, the 90-Percent Rule is a principal fundamental in photography that can help you fine-tune your exposures of your photography to accurately depict the true skin-tones of your subject.

(April 2006 Playboy Playmate Holley Dorrough photographed at my International Glamour, Beauty & the Nude photography workshop in Philadelphia, Oct. 2006)