Bulb mode is a camera setting that keeps the shutter open for as long as the shutter button or a remote trigger is held, rather than for a fixed, pre-set duration.
Most cameras stop their timed shutter range at 30 seconds. Anything longer needs Bulb mode, where the shutter stays open for as long as the release is held. That covers a lot of real work: a 10-stop ND filter on a midday cityscape can call for a two-minute exposure, a single-frame star trail might run twenty minutes, and light-painting a building interior often takes several minutes of walking around with a torch.
Bulb also removes the guesswork once filter stacking pushes the exposure past what a meter can read. The catch is that the camera cannot meter or time the shot for you, so Bulb work depends on an external timer and a locking remote or intervalometer. Starting and stopping a two-minute exposure by hand on the shutter button usually adds enough vibration to soften the first and last seconds of the frame.
Planning a 4-minute exposure of a lighthouse at dusk with a 10-stop ND filter, the metered reading without the filter is 1/4 second at f/8, ISO 100. Ten stops slower is about 4 minutes, which no shutter dial offers, so the camera goes to Bulb. A locking cable release or an intervalometer set to 240 seconds opens and closes the shutter without anyone touching the body.
Across those four minutes the sky keeps darkening, so an exposure that was correct at the start reads slightly bright by the end, and many photographers cut the planned time by thirty seconds to compensate. For a longer star-trail single frame, the same Bulb setup runs 15 to 60 minutes, with in-camera long-exposure noise reduction then doubling the wait before the file is written.
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