Metering

Light meters measure the amount of light. Then, for a given film speed, (ISO) they calculate what settings the camera needs to be on to create a perfectly exposed photo. Light meters are designed to measure for middle gray. This means that the camera after choosing a middle gray will create highlights and shadows based on the where the middle gray lies.

Most digital cameras now days have built in light meters, that are activated when pressing the shutter half way down. (at the same time you are focusing, you are reading the lighting) In regular lighting the camera does a pretty good job of metering correctly. However, sometimes lighting plays tricks on the camera. This is often found when there is a really bright background. The camera meters for the sky, leaving the subject dark. As in the example above. The camera automatically measured the background as middle gray, leaving the subject in shadows. The photographer then metered specifically on his subject so that the camera knew where to measure the middle gray from. The second picture shows the subject correctly metered for, and the background a little lighter then before.

We practiced this concept in class using our computer screens. We first metered for the computer screen, and second metered for our partner. As you can see, the first picture shows the computer screen being perfectly exposed, however that left the rest of the room way below middle gray so it is all shadows. Then when the camera reread the light on our partners skin, our partner became the middle gray, so then the scale shifts. The screen is no longer blue, but overexposed, but our partners become visible in the darkness.
It is not always necessary to meter, but when the situation arises, it can make all the difference.