Cable release

Bulb, abbreviated B, is a shutter speed setting on an adjustable camera that allows long exposure times under the direct control of the photographer. With this setting, the shutter simply stays open as long as the shutter 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.

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:

BULB EXPOSURE (B). Another term for a brief exposure—in which the shutter remains open only so long as the shutter release is held down. The word originated with the early pneumatic shutter release.

It is not clear when the term actually originated. The first industrial produced leaf shutters from Voigtländer in Germany in 1890 had two settings: "M" for Moment exposure and "Z" for Zeit (English: time) exposure. The M-setting enabled the adjustable fractions of a second.

With the Z-setting the shutter opened when pressed and closed when it was triggered again. Around 1894 the third setting "B" for Beliebig (meaning beliebige Zeit, English: any time) was added for the shutter of C. A. Steinheil & Söhne in Munich. The B-setting enabled his shutter to stay opened as long as the release was pressed. Bausch & Lomb in Rochester overtook these settings for their popular Unicum shutters in 1898. With the worldwide success of the shutters of Wollensak like the Junior since 1903, most of the other shutter manufacturers in the world overtook the (now international) settings T for time, I for instantaneous. The B setting was kept until today for the familiar bulb behavior that could be achieved by a cable release in the English-speaking world and for Beliebig in Germany.

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

Because of the risk of camera movement, the camera is most often mounted on a tripod 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.

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Wikipedia