Lightmeters, metering and meter modes
The thing to know/remember about light meters is they all read light as a medium gray.
They do this in two basic ways. Light reflected from the subject; or Light falling on the subject. (called incident light)
Camera Meters
The light meters in our cameras are all reflected light reading meters.
Metering Modes
The meters in our cameras handle the light readings in two or more ways. The most common are "Center Weighted Averaging" "Spot Metering"(called partial in Canons) , and "Matrix" or "Evaluative Metering".
Center weighted averaging is what it sounds like, The light sensors in the camera are averaged for the entire frame and more "weight" is given to the center third of the frame.
Center weight metering is good for wider scenes with fairly even lighting.
Spot metering, also called partial metering in Canons, reads a much smaller portion of the frame. From 9% to less than 4%. This mode is almost always locked to the center of the frame, and the area used indicated by one or more circles in the center of the viewfinder.
Spot metering is useful for scenes where your subject is in much different light than the background. A person in a spotlight on stage, or posed in front of a bright window might benefit from spot metering.
Matrix or Evaluative metering is the most sophisticated , using the entire camera system to set the proper exposure. Matrix reads each of the light sensors in the camera, takes into account the selected focus point and the distance to the subject, then compares all that to a database of images; then it sets an exposure that is closest to the most matches with the database.
Evaluative metering is a key component in the camera flash system, especially with the system flashes from Canon and Nikon.
Hand Held Light Meters
A hand held light meter is the tool that can take your photography to a more sophisticated level.
Most modern hand held meters can measure both reflected light readings and incident light readings. This is usually done by sliding a small diffuser over the light sensor on the meter. Many will also make readings of a flash or strobe exposure. (the Sekonic L308s is a small multi-mode meter that will not break the bank.)
Light Readings
Reflected light readings measure the light reflected from the subject, and are made from the camera position by pointing the meter or camera at the subject and activating the meter with a half-push of the shutter or a full push of the meter switch.
Reflected light readings are good when your subject is moving in relation to a light source. (closer or farther, or in and out of shade for example)
Reflected light readings can be influenced by the relative light or dark of the subject.
In the studio for example, a "high-key" set would cause a reflected light reading to indicate an under exposure; the meter would "see" all that white as "light".
Incident light readings measure the light falling on the subject, and are made from the subject position. The meter is held at the subject, and pointed toward the camera position.(You may have seen this method on TV or in a movie, it looks cool and technical)
Incident light readings are useful when your subject to light distance remains constant.
And because the meter is the "gray point" the relative lightness or darkness of the subject wont influence the exposure.