Metering allows you to identify the exposure triangle you need to set to get the output you want in your final picture.
Because there is no one size fits all in photography. Metering allows you to understand the light in the picture you want to capture. You can use a light meter created for the purpose, a phone app (there are some surprisingly good ones available on the iPhone), or the camera itself.
There are three types of metering:
- Incident metering.
In incident metering, the meter is held by the person or object being photographed and the measurement is taken of the incidental light falling on the subject.
- Reflective metering.
As the name suggests, in reflective metering, the light meter is pointed at the scene to be photographed and measures the light reflected from it.
- Spot metering
Spot metering is a type of reflective metering that meters a very specific area of the field of view. Spot metering allows you to meter part of the image only.
All modern cameras use reflective metering to identify the best exposure for an "average" scene (which is determined by a value called 18% grey or mid-grey). Good DSLRs also have the ability to spot meter, which allows you to ensure the area of the photograph you care about most is properly exposed.