Bulb (photography)
Encyclopedia
Bulb, abbreviated B, is a shutter speed
Shutter speed
In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....

 setting on an adjustable camera that allows for long exposure times under the direct control of the photographer. With this setting, the shutter
Shutter (photography)
In photography, a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene...

 simply stays open as long as the shutter release button remains depressed. An alternative setting common on film cameras is Time, abbreviated T, where the button is pressed once to open the shutter and again to close it.

History

The term bulb is a reference to old-style pneumatically actuated shutters; squeezing an air bulb would open the shutter and releasing the bulb would close it. According to the Focal Encyclopedia of Photography:
It is not clear when the term actually originated. It appears that when instantaneous shutters were introduced, they included a B setting so that the familiar bulb behavior could be duplicated with a cable release. This may have been the origin of the term bulb in reference to shutter settings. (See the photo captions.)

Use

The bulb setting is used on some cameras, including some point-and-shoot cameras, to obtain shutter speeds slower than the maximum offered by the camera otherwise.

Because of the risk of camera movement, the camera is most often mounted on a tripod
Tripod (photography)
In photography, a tripod is used to stabilize and elevate a camera, or to support flashes or other photographic equipment. All photographic tripods have three legs and a mounting head to couple with a camera...

 for the duration of the exposure. While it's generally possible to use the shutter release button on the camera itself, a cable release or electronic remote is often used to further eliminate the risk of shaking the camera during the long exposure. The cable releases generally include a locking feature to eliminate the need to keep the button or plunger depressed during extremely long exposures.

The bulb setting is useful for the following types of photographic subjects:
  • fireworks
    Fireworks
    Fireworks are a class of explosive pyrotechnic devices used for aesthetic and entertainment purposes. The most common use of a firework is as part of a fireworks display. A fireworks event is a display of the effects produced by firework devices...

     at night
  • the night sky
    Night sky
    The term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...

     and celestial objects (see astrophotography
    Astrophotography
    Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...

    )
  • lightning
    Lightning
    Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

  • streets at night (creating streaks from moving cars)

On modern cameras

On some modern cameras, bulb is a mode available as an option on the LCD menu only, if possible at all. On others, including many Digital SLR cameras, bulb is available from the manual exposure mode or shutter priority modes.

When set to bulb, generally on the "M" or manual setting of the camera, the shutter
Shutter (photography)
In photography, a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene...

 will stay open as long as the shutter release button (or shutter release cable or remote) remains depressed.

Some mid-level or bridge cameras
Bridge digital camera
Bridge cameras are cameras which fill the niche between the single-lens reflex cameras and the Point-and-shoot camera. They are often comparable in size and weight to the smallest Digital SLRs , but almost all digital bridge cameras lack an optical viewfinder system...

 such as the Olympus SP-560UZ have a 'limited' bulb setting, allowing time exposures up to 8 minutes and at an ISO setting
Film speed
Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system....

 of 200 ISO and lower.

Some specialized cameras use other image trigger
Image trigger
An image trigger initiates the capture of single or multiple frames of a digital camera by analysing the signals of its sensor.For capturing and analysing of fast moving objects An image trigger initiates the capture of single or multiple frames of a digital camera by analysing the signals of its...

s.
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