Nikon D5100 User Manual page 174

4
Open the shutter.
Bulb: After focusing, press the shutter-release button on the camera or optional
remote cord all the way down. Keep the shutter-release button pressed until
the exposure is complete.
Time: Press the ML-L3 shutter-release button all the way down.
5
Close the shutter.
Bulb: Take your finger off the shutter-release button.
Time: Press the ML-L3 shutter-release button all the way down. Shooting ends
automatically after thirty minutes.
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