The Mechanics of Metering

Your camera works on the principle of a reflective meter, which means it measures the light reflected off the subject and scene. Unlike a handheld or incident meter, which meters the light directly falling onto the metering cell, the meter in a DSLR is based entirely on the light being reflected by varying degrees off the different elements within the scene.

Every camera meter is calibrated for an average reflectance. Traditionally, it has been said that each camera meter is calibrated for an average reflectance of 18 percent gray, which is the halfway point between black and white. However, to be more precise, camera meters are actually calibrated for a standard reflectance of 12 percent. Now, that may be a key concern for techies, but that 6 percent difference doesn’t provide a practical impact for me.