Every camera today has a built in metering system, but what is "metering"? Metering light means to take a measurement of how much light is either reflecting from a scene / object or how much light is falling on the scene.
There are two kinds of Light meters. Reflective meters (hand held spot meters and the metering system in your camera take reflective readings) and incident meters which measures the light falling onto the scene. In today's digital cameras you normally have three different reflective metering options - spot, center weighted or evaluative/matrix. Spot metering inside your camera works just like a hand held spot meter. It takes a reading from 2-4 degrees from the very center point of your camera. Center weighted metering takes an average reading from a larger portion of the center of the camera. Matrix / evaluative readings sample from up to 100 or more areas and then average them.
When you meter the light with your camera's in camera metering system or a hand held light meter you are taking a reading of how much light is reflected from your subject. This reading will render a perfectly balanced middle gray image. It will be balanced in terms of values, not in terms of aesthetics.
On the other hand, an incident light meter reads the light falling on the object regardless of the brightness or density of the object. Although this may seem like a more ideal way of working and it probably is, there are fundamental problems with this method. It truly only takes the total volume of light into consideration and not the light ratios or the reflective nature of the scene. It also falls apart when working with a back lit subject.
In our next blog post we will discuss middle gray.