When using the substitution method, you replace an object within the scene with an object of known reflectance, such as a Kodak Gray Card and take a reflected-light meter reading from this object.
You can also substitute objects that match the light reflectance quality of the object in the scene. Don’t have a gray card? Back in the film days I used to take a meter reading of grass (if there was any in the scene) and just open up one stop but I don’t remember the last time I tried this old “rule of green thumb.”
©2006 Joe Farace
Red or gray?
Fifty years ago, a group of investors led by film actor Johnny Weissmuller (aka Tarzan) created a hotel called Los Flamingos that was located on Acapulco’s high cliffs. Areas such as this lanai capture those feelings today and if the porch isn’t 18 percent gray it’s close enough for a substitute reading of 1/80 sec at f/4.9 at ISO 200.
The limitations of metering When pointed at subject, reflected light meters, whether in-camera or handheld, are calibrated to give an accurate exposure with reflectivity somewhere around 18%; the exact value varies and the details are complex, with some meters measuring 12% (the most common) and others at 14%. 
By placing a Kodak Gray Card in the scene to be photographed and taking a reading off of it with a light meter, the photographer can be assured of consistent exposures, but sure be sure to read the fine print include with the card instead of just accepting the reading as correct. The instructions packed with the card contain the following advice about adjusting the meter readings taken with the gray card:
Normal subjects: “Increase the indicated exposure by 1/2 stop.”
Light subjects: “for very light subjects decrease exposure by 1 stop”
Dark subjects: “If the subjects is dark or very dark increase the indicated exposure by one to one and one-half stops.”
Joe Farace is co-author of “Better Digital Available Light Photography” along with Pulitzer Prize winning photographer Barry Staver. It was published by Focal Press and is available in all the best bookstores, including Barnes & Noble and Amazon.com.
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