I often get questions about light metering, and the confusion between “incidental” and “reflected” light metering. I was responding just this morning to a question via email and thought I’d go ahead and include my answer here as well, in case this might help any of you!
Reflected Light Meter: Measures reflected light. This means you point the meter at the scene being photographed. This measures the brightness of the scene you are looking at, and comes up with exposure values so that the photograph created will be the right brightness. Unfortunately, this approach does not know whether the scene is a black wall or a field of snow. The reflected light meter just makes an assumption about how bright the resulting elements in the photograph typically should be, often in some middle gray value because that is generally true, though not always true. Many sources say Ansel Adams came up with the 18% gray as being the perceived brightness of a middle gray value. This is somewhat subjective, but that became the (approximate) standard. The problem with that approach is that the meter has absolutely no idea how bright the photo should ACTUALLY be, so it just assumes most of the time, the main elements of the photograph are somewhere in the middle gray range, not white, not black. The meter therefore assumes the point of reference of 18% gray is the right brightness for the photograph. Since it is calibrated based on that assumption, it provides shutter speed and aperture value recommendations so that the thing being metered turns out to be mid gray. So, if you are metering off a target that is 18% gray, then the meter will give you a pretty accurate exposure setting so that the gray target turns out gray, and everything else in the photo that is not gray but is in the same light will turn out to be the right brightness as well, white will be white, black will be black, gray will be gray, as long as you meter off the thing that is gray. That is where the camera’s different “Metering Modes” come into play, because you can control where the camera meters on elements in the scene since that is critical for reflected light metering. This is also why cameras have settings like “Exposure Compensation” to allow you to adjust when the camera’s reflected light metering gets the answer wrong. Also electronic flash (Speedlight) TTL metering is based on reflected light metering, therefore handheld incidental light metering is not involved in TTL flash photography.
