Canon EOS M Basic Instruction Manual page 72

Changing the Focusing Method
Using Continuous AF
Using AF+MF Mode (Full-time manual focusing)
Set the Focus mode to [AF+MF].
After focus is achieved using AF, you can adjust the focus manually as
desired using the focusing ring while holding the shutter button pressed
halfway.
However, in <5> mode (p.59) or if [AF operation] is set to [Servo AF]
(9?), priority is given to Servo AF. In such cases, even if you set the
focus mode to [AF+MF], you will not be able to focus manually.
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The default setting is [Enable].
Since the focus is constantly close to the
subject, when you press the shutter button,
the camera quickly focuses the subject. If
[Enable] is set, the lens will operate
frequently and consume more battery
power. This will reduce the number of
possible shots (battery life).
When an EF or EF-S lens is used, during
Continuous AF, turn off the power before
setting the lens's focus mode switch to
<MF>.