Canon EOS M Basic Instruction Manual page 250

a: Manual Exposure
Using Speedlite 90EX
To obtain a correct flash exposure for the main subject, the flash output
will be set automatically to match the set aperture (autoflash exposure).
You can set the shutter speed from 1/200 sec. to 30 sec. or bulb.
BULB: Bulb Exposures
Since bulb exposures produce more noise than usual, the image may
look a little grainy.
When [A4: Long exp. noise reduction] is set to [Auto] or [Enable],
noise generated by the long exposure can be reduced (p.161).
For bulb exposures, using a tripod is recommended.
You can also use a Remote Controller (sold separately, p 291) for bulb
exposures. When you press the Remote Controller's transmit button, the
bulb exposure will start immediately or 2 sec. later. Press the button
again to stop the bulb exposure.
150
A bulb exposure keeps the shutter open
for as long as you hold down the shutter
button. It can be used to photograph
fireworks and other subjects requiring
long exposures.
As explained in step 3 on page 148,
either tap [Y] or turn the <5> dial to the
left to select [BULB], then press the
shutter button. The elapsed exposure
time will be displayed on the LCD
monitor.