Modern digital cameras offer a number of metering modes – typically evaluative metering, partial metering, spot metering and centre-weighted average metering.
This mode goes back to the early days of in-camera metering. The light reading is averaged across the scene but with more weight given to the centre. The reading may be fooled when the subject is surrounded a very bright or dark background but it is easy to understand what the meter is doing and so it is relatively easy to apply manual exposure compensation.
When the background is much lighter or darker than the main subject, setting partial metering causes the camera to meter just a small area in the centre, typically about 6% and usually indicated by a circle in the viewfinder. Placing this area over the subject allows a light reading just from the subject, ignoring the background. This is a very useful mode. Optionally, lock the exposure using auto exposure lock and recompose before taking the shot.
Spot metering is similar to partial metering except that the central metering area is reduced further to 1–2% of the total, allowing a very specific part of the subject to be measured.
Evaluative or multi metering
Evaluative metering tends to be the default setting for modern cameras but it is also the most complex. The camera takes multiple light readings from around the scene and then analyses them to try to assess where the subject is in the frame and what the best overall exposure setting might be.
Some modern cameras have a face-detection capability, which seeks to detect human faces within the frame and then optimises the exposure (and focus) for the faces.
Both partial and spot metering can be useful when there is a very large variation in brightness across the frame – for example a landscape with bright sunlit areas and deep shadow areas. By spot metering both the bright and the shadow areas, a better analysis can be made of what exposure to use.
Which mode to choose
In my experience evaluative metering works just fine 90% of the time and can be used as a fairly reliable default setting. For portraiture or wildlife photography, where it is necessary to get the exposure just right on a specific area, partial metering is a good option. For street photography at night, where light levels can be all over the place, I prefer centre-weighted average, where the camera won’t try to be too clever and it is easier to understand what is going on and compensate if needed.