Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 116)

bulb: Long (Bulb) Exposures
In this mode, the shutter stays open as long as you hold down the shutter button completely,
and closes when you let go of the shutter button. Use bulb exposures for night scenes,
fireworks, astrophotography, and other subjects requiring long exposures.
1.
Set the shooting mode to < >.
Press the < > button and turn the < > or < > dial to
select < >.
2.
Set the desired aperture value.
Watch the top LCD panel as you turn the < > or < > dial.
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