Metering is the way your camera measures the amount of light that enters the camera and reaches the sensor, determining the exposure. On a digital SLR this measurement is displayed via a light metre that can be seen by a photographer through the viewfinder. It has a scale that ranges from -2 (underexposed) to 2 (overexposed), with 0 at the centre.
You can control the amount of light that enters the camera, by selecting a suitable shutter speed, aperture and ISO on manual mode. Once you’ve done this, a blinking needle on the light meter will show you where the exposure falls when you hold down the shutter button without pressing it all the way.
However this light metre works off a digital formula that aims for 18% grey - which is basically averaging out the white and black tones (or the darkest and brightest points) across the entire photograph. A photograph that has a reading of 0 on the light metre might be considered to be properly exposed but it won’t necessarily look like you want it to. This usually happens when there are more bright tones than dark tones in a scene, or vice versa. When this happens images with a lot of white or black tones will look grey, such as with the white on white and black on black photographs below.
This is why it helps to choose a metering mode that's suitable for the type of light you are shooting in. On most SLRs there are three or four different metering modes to choose from. They all measure light in different ways, as explained below:
1) Evaluative Metering (Canon) or Matrix Metering (Nikon)
This metering mode measures the light of the entire scene as divided into zones. It gives highest priority to the zone in which your AF point (focal point) is located. It’s great for evenly lit scenes and is a good default mode.
2) Centre-weighted Metering
This mode doesn’t factor in the location of the AF point, instead measuring the light within the centre of the frame. It’s good for when you have a subject located in the middle of a frame who won’t move, such as when shooting food or still life images.
3) Spot Metering
This mode measures only a small area of light surrounding your AF point and ignores the light in the rest of the scene. This is a great mode to use when working in a location with contrasting tones such as a light corner in an otherwise dark scene, when shooting backlit subjects and to get the skin tones right when shooting portraits by metering on the skin.
4) Partial Metering (available only on Canon cameras)
This mode measures a slightly larger amount of light around the AF point similar to Spot Metering.
If after choosing a suitable metering mode, you’re still not getting the results you want, you may need to use Exposure Compensation.
What is exposure compensation?
Exposure compensation is an option where the light meter needle can be moved up or down from the automatic setting of 0, in increments of half stops from zero to a positive value (up to 2) for more light or to a negative value (down to -2) for less light, in order to get the desired level of exposure to suit the scene.
It is best to use exposure compensation when you’ve set your shutter speed, aperture, ISO and metering mode and the exposure is still not quite right.
Take a look at the images below to see the difference that exposure compensation can make. Both the images reflect the true colours and tones in the scene after exposure compensation has been used.