Canon EOS 5D Mark III Instruction Manual (en)

When bulb is set, the shutter stays open as long as you hold down the 
shutter button completely, and closes when you let go of the shutter 
button. This is called bulb exposure. Use bulb exposures for night 
scenes, fireworks, the heavens, and other subjects requiring long 
Set the Mode Dial to <F>.
Set the desired aperture.
While looking at the LCD panel, turn 
the <6> or <5> dial.
Take the picture.
While you hold down the shutter 
button, the exposure will continue.
The elapsed exposure time will be 
displayed on the LCD panel.
F: Bulb Exposures
Elapsed exposure time
Do not point the camera toward an intense light source, such as the sun 
on a sunny day or an intense artificial light source. Doing so may 
damage the image sensor or the camera’s internal components.
Long exposures produce more noise than usual.
If ISO Auto is set,  the ISO speed will be fixed at ISO 400 (p.127).
When [z3: Long exp. noise reduction] is set to [Auto] or [Enable], 
noise generated by the long exposure can be reduced (p.146).
For bulb exposures, using a tripod and Remote Switch RS-80N3 (sold 
separately) or Timer Remote Controller TC-80N3 (sold separately) is 
recommended (p.187).
You can also use a remote controller (sold separately, p.188) 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.