There is a special setting on all DSLR cameras that allows the photographer to go beyond the longest automatic shutter speed setting of 30 seconds. In fact, this special setting allows infinitely longs exposures to be made. This is the Bulb setting, or Bulb exposure mode.
The shutter speed range on a DSLR camera finishes at 30 seconds. One click below that selection is Bulb, which allows the photographer to manually hold the shutter open for as long as is required as the shutter button is pressed down. The camera bulb exposure setting is only available in Manual mode.
The Bulb mode is basically a creative feature. It allows shots to be taken that are minutes long, rather than seconds. A common image to capture is of the trails of vehicle lights when it is dark. These moving lights will create artistic streaks of white and red patterns emitted from the head lights and tail lights of passing traffic. If you are near a traffic junction you may capture orange streaks from indicator lights. And if you’re lucky enough, you may get a streak of blue from the flashing light of an emergency vehicle.
Two things you must do in order to shoot effectively in Bulb mode:
Use a Tripod: Attach the camera to a tripod in order to ensure that no camera movement will ruin the shot. You don’t have to use a tripod so long as the camera is on a solid base, like a wall. Turn off the any image stabilization mechanisms you may might have on your camera or on the lens.
Use a Remote Shutter Release: A remote shutter release will prevent vibrations ruining the shot as you press the shutter button. No matter how careful you are, you will always cause a tiny bit of vibration as you depress the shutter button. That doesn’t matter with fast shutter speeds, but in Bulb mode you will record vibrations, however small. If you don’t have a remote cable, use the camera’s built-in self-timer delay, which is just as effective. A two second delay will be adequate.
History of the Bulb Mode:
Decades before the first flashbulbs, some box cameras and many view cameras and folding cameras came with a detachable pneumatic shutter release with a rubber bulb on the end; “Bulb” refers to the rubber shutter release bulb. Though mechanically timed exposures too could be triggered by squeezing the shutter release bulb, “Bulb” exposures then had the same momentary action as camera shutters have today.