Canon EF-M - Manual (Page 50)

In situations where there is a large differ­
ence in brightness between the main sub­
ject and background, the subject may not
be properly exposed depending on the
composition. In such situations use the
partial metering button to selectively
meter the desired portion of the scene
and lock the exposure setting. When par­
tial metering is used the exposure will not
change even if the composition is
changed.
1) Compose the picture and focus the
subject.
2) While pressing the shutter button half­
way, cover the subject with the view­
finder's partial metering mark and
press the partial metering button.
* lights in the viewfinder’s infor­
mation display to indicate that the
camera is in partial metering mode.
Once appears you can release
the partial metering button. The ex­
posure is locked.