Canon EOS M Instruction Manual page 149

In [A3: Auto Lighting Optimizer], if the checkmark <X> for [Disable
during man expo] is removed, it can be set in the <a> mode (p.159).
When ISO Auto is set, you can press the <YA> button to lock the ISO
speed.
After recomposing the picture, you can see the exposure level difference
on the exposure level indicator (p.24) compared to when you pressed the
<YA> button.
5
Set the exposure and take the
picture.
Check the exposure level and set the
desired shutter speed and aperture.
If the exposure level exceeds ±3
stops, the <I/J> will light up at <-3>
or <+3> on the exposure level scale.
a: Manual Exposure
149