Time to read the meter
Do you know how your Canon’s exposure metering modes really work?
EOS cameras feature a number of exposure metering patterns: Evaluative, Centre-weighted, Partial and Spot. Choosing the right one is a mixture of personal preference, knowledge and experience.
Evaluative metering is the default and uses a number of segments covering almost the whole frame.
It is intelligent enough to identify medium backlit subjects and avoid underexposure on beaches and in the snow. When paired with One Shot AF there is an automatic exposure lock applied when focus locks. This means that if you are in the habit of using the centre AF point to focus, then the exposure is locked before you recompose. For AI Servo AF there is no automatic exposure lock without customizing the camera settings.
Centre-weighted, Partial and Spot never apply exposure lock, and can cause problems for the unwary; if you focus, lock and recompose there is a likelihood of exposure errors.
Spot metering is the smallest active zone, typically two per cent at the centre of the frame. EOS-1D cameras can be configured to link the Spot meter to the selected AF point. Partial metering takes a reading from a central zone covering less than 15% of the frame. Centre-weighted uses more of the frame, but places the majority of emphasis at the centre.
The meter measures the brightness of the light reflected from the subject. I find Evaluative metering on the 7D Mk II and 5D Mk IV to be an improvement over the previous models, and I rely on this mode in most situations.
For heavily backlit portraits, you can use Partial metering, but need to lock the exposure before recomposing. It is also effective for motorsports where you measure the light reflected from the racetrack, as it is often close to mid-grey. Then your shots won’t be affected by the colour of the cars.