Bulb mode is a shutter setting in which the shutter stays open for as long as the shutter release is held down and closes when it is released. In photography it is the standard way to make exposures longer than the longest timed speed of the camera, which on most interchangeable-lens cameras is 30 seconds.
In camera mechanics, bulb mode is marked B on the shutter speed dial or in the mode menu. The name comes from early pneumatic shutters that stayed open while the photographer squeezed a rubber bulb. Today it is used for night photography, astrophotography, fireworks, light painting and long exposures with dense neutral density filters.
How bulb mode controls the shutter
In a timed exposure, the camera’s own clock decides when the shutter closes. In bulb mode the photographer becomes the timer: pressing the release opens the first curtain, and releasing it triggers the second curtain to close. The camera records no fixed shutter speed until the exposure has ended.
Because holding a button by hand shakes the camera, bulb mode is almost always combined with a tripod and a remote release. A wired release with a locking button, an infrared remote, a smartphone app or an intervalometer keeps the shutter open without touching the body. Several related modes exist on modern cameras.
| Mode | How it works | Typical use |
|---|---|---|
| Bulb (B) | Open while the release is held | Exposures of unknown or variable length |
| Time (T) | First press opens, second press closes | Long exposures without a locking remote |
| Bulb timer | Camera closes the shutter after a preset duration | Repeatable long exposures |
| Timed speeds | Camera clock sets the duration | Exposures up to about 30 seconds |
| Live composite | Camera adds only new bright areas over time | Star trails, light painting with live preview |
Exposure times and how to calculate them
Bulb exposures are planned by counting stops from a metered test shot. A common method is to take a test frame at a high ISO, where the exposure is short, then convert it to the target ISO by doubling the time for each stop of ISO reduction.
| Test exposure | Change | Bulb exposure |
|---|---|---|
| 15 s at ISO 6400 | Reduce to ISO 400 (4 stops) | 240 s, or 4 minutes |
| 30 s at ISO 1600 | Reduce to ISO 100 (4 stops) | 480 s, or 8 minutes |
| 1/30 s, no filter | Add a 10-stop ND filter | About 32 s, use bulb |
| 1/8 s, no filter | Add a 15-stop ND filter | About 68 minutes |
Very long exposures may not follow the arithmetic exactly. Digital sensors behave linearly, but film suffers from reciprocity failure, which demands additional time beyond the calculated value. With digital cameras, the main departures are thermal noise and battery drain rather than exposure errors.
Light that changes during the exposure makes calculation harder. At dusk the ambient level falls by roughly one stop every few minutes, so a bulb exposure started at the end of blue hour may need to be lengthened, and the next frame must be recalculated. In light painting or fireworks, the duration is set by the event rather than by the meter, and aperture and ISO are chosen to keep the moving light sources from clipping.
Bulb mode and other camera mechanics
The behavior of bulb mode depends on the shutter type and on other mechanics of the camera body. These interactions explain why the same exposure can look different on different cameras.
| Shutter type | Behavior in bulb | Consideration |
|---|---|---|
| Mechanical shutter | Curtains open and close physically | Small vibration at start; counts toward shutter count |
| Electronic first-curtain shutter | Exposure starts electronically | Eliminates start vibration, negligible at long times |
| Electronic shutter | Sensor readout ends exposure | Not available in bulb on many cameras |
| Global shutter | Whole sensor starts and stops together | No rolling effects; maximum time depends on model |
Image stabilization should usually be turned off on a tripod, because some systems can introduce drift when no movement is present. Some cameras also limit the maximum bulb duration, for example to several minutes or hours depending on the model, and many disable the electronic viewfinder preview during the exposure.
Noise, heat and long exposure noise reduction
Long bulb exposures heat the sensor and accumulate dark current, which appears as hot pixels and a faint glow in the corners. Cameras address this with long exposure noise reduction, which takes a second frame of equal length with the shutter closed and subtracts its pattern from the image.
The drawback is time: a five-minute exposure requires another five minutes of dark frame capture, which halves the number of frames possible during a short twilight. Astrophotographers often disable the in-camera function and capture a set of separate dark frames later instead, to be subtracted during stacking.
Common mistakes in bulb mode
Bulb exposures fail for simple reasons more often than for technical ones. The most frequent problems concern stability, stray light and preparation.
| Problem | Cause | Remedy |
|---|---|---|
| Blurred image | Pressing the release by hand or wind on the tripod | Remote release, weighted tripod |
| Light leak or fog | Open eyepiece on an optical viewfinder | Close the eyepiece shutter or cover it |
| Frame ends early | Battery runs out during exposure | Fresh battery or external power |
| Unsharp stars or lights | Autofocus searching in darkness | Focus manually and switch autofocus off |
| Strong hot pixels | Warm sensor, long exposure | Dark frame subtraction |
It also helps to time the exposure with a separate clock or the bulb timer function, because counting seconds in the dark is unreliable, and to check the histogram after the first frame rather than judging brightness on the screen at night.
Understanding it simply
A camera usually decides by itself how long to let light in, often only a fraction of a second. Bulb mode hands that decision to you: the shutter stays open for as long as you keep the button pressed.
This is useful at night or for special effects, where you may need several seconds or even minutes of light. The camera must stand perfectly still on a tripod, and a remote button helps, because even a gentle touch would blur the picture.
Applied in practice
A photographer covers a city fireworks display from a bridge. She sets the camera on a tripod, focuses manually on the first burst, and chooses ISO 100 at f/11 to keep the bright trails from overexposing.
With the camera in bulb mode and a wired remote, she opens the shutter as each rocket launches and closes it after two or three bursts, which gives exposures of about 4 to 12 seconds. Holding a black card in front of the lens between bursts lets her combine several explosions in one frame while keeping the sky dark. The result is clean, layered fireworks trails above the illuminated skyline, with no camera shake.