This tutorial looks at what the different metering modes; Evaluative, Partial and Centre-weighted average metering modes, do. When we press the shutter button half-way down our camera gets a focus lock and works out what is the best exposure for the scene. If we are in Av mode this means deciding on the shutter speed. If we are in Tv mode this means deciding on the aperture size.
Evaluative metering is the most commonly used metering mode. When you take a picture in this mode the camera 'looks' at the whole scene and works out the average appropriate exposure. The aim of this is to get as close to the correct exposure as possible for the whole scene.
Partial metering sets the exposure to be correct for whichever auto-focus point has locked on and ignores the rest of the scene. The aim is to have the subject (which you have focused on) correctly exposed and not to worry about anything else.Centre-Weighted Metering
Centre-Weighted average metering sets the exposure correctly for the center of the frame and averages out the rest of the frame. The idea of this is that your subject in in the centre of the frame so it prioritises the exposure for the centre. However it will also try to balance the exposure so that the rest of the frame isn't too over or underexposed. This is useful when the background to our subject is of interest as well.
This pictures deomnstrated the difference between the 'Evaluative' and 'Partial' metering modes. The picture on the left was taken with evaluative metering and the picture on the right was taken with partial metering. Against the bright light the subject is slightly under-exposed with the background being exposed properly when taken with evaluative metering. When partial metering mode is selected the subject exposed correctly but the background and the highlights are slightly over exposed.
Partial metering is useful when shooting a subject against a bright background, where the correct exposure of the subject is more important than the background.