Canon EOS M EF-M 22mm STM Kit Instruction Manual page 69

With Speedlite 90EX Attached
(When you use flash, there is a slight time lag after you press the shutter button
completely before the picture is taken.)
The flash fired even though it was daylight.
For a backlit subject, the flash may fire to help lighten the subject's
dark areas. If you do not want the flash to fire, switch the Speedlite's
power to <2>.
The flash fired and the picture came out extremely bright.
Move farther from the subject and shoot. When shooting flash
photography, if the subject is too close to the camera, the picture
may come out extremely bright (overexposure).
The flash fired, but the photo came out dark.
You were too far away from the subject. See "Effective Flash Range"
(p.139) and shoot.
When flash was used, the bottom part of the picture came out
unnaturally dark.
The subject was too close to the camera, and a shadow was created
by the lens barrel. The subject should be at least 1 meter/3.3 feet
away from the camera. If a hood is attached to the lens, remove it
before taking the flash picture.
A Fully Automatic Shooting (Scene Intelligent Auto)
(p.138)
69