A digital camera’s meter is the part of the camera that calculates the exposure for your photograph, i.e. the correct settings for shutter speed and aperture, based on the light conditions, and the ISO that you have set.
Many cameras have a choice of different metering modes, and these affect the way in which the camera’s meter will make its calculations. An understanding of how these modes work can increase your chances of getting the optimum exposure for your photographs.
Camera meters measure reflected light, i.e. light which hits the subject and is then reflected off it. However, different subjects reflect different amounts of light. The meter therefore assumes that all subjects are a mid-tone grey, and calculates the exposure based on the reflected light from a mid-grey subject.
If you’re photographing a scene which contains a range of dark and light tones which would average to a mid-tone grey, then the camera’s meter will have no problem giving you the correct exposure. But in scenes with predominantly dark or light tones, or unusual lighting situations such as back light or high contrast, this approach can lead to over or under exposure. It’s in these situations when it’s helpful to know which of the metering modes will give the best result.
Evaluative Metering/Matrix Metering
This is usually the default or standard metering modes, and the name will vary from one camera manufacturer to another. This exposure mode uses algorithms that evaluate multiple segments of the scene, taking into account factors including contrast, brightness, subject distance, and subject colour. This will produce an accurate exposure in a scene which contains a range of dark and light tones which would average to a mid-tone grey.
In this mode the camera still meters the entire frame, but assigns the greatest weight to an area in the centre of the frame. So this would be the mode to choose when your subject is in the centre of the frame, and you don’t want the exposure to be affected by very dark or bright areas around the edges.
This mode is not available on all makes of camera, and is mostly found on Canon cameras. It meters around 10-15% of the frame (a larger area than spot metering – see below). It is useful when your subject is relatively small within the frame, and is against a much lighter or darker background.
This is a metering mode which allows very precise metering from just a small area of the scene, between 1-5% of the frame. Sometimes this reading is taken from the centre of the frame, in which case you will need to recompose after metering if your subject is off centre; but often the camera will have an option to select a different part of the frame for taking the spot metering reading. Both spot metering and partial metering would be good modes to choose when taking a portrait of a person who is backlit – metering from their face will give a more accurate exposure, preventing them from becoming silhouetted against the bright background.