Bulb mode, marked B on the mode dial or just past 30 seconds at the slow end of the shutter speed list, holds the shutter open for exactly as long as you hold the release down. Most cameras count their own shutter speeds only up to 30 seconds, so B is how you make an exposure of a minute, ten minutes or an hour: the camera stops counting and you take over.
That one change has three consequences, and this page is about all three. You need a way to hold the shutter open without touching the camera. You need a way to time the exposure. And because the meter cannot suggest a time it cannot set, you need to work the exposure out yourself, which turns out to be simple doubling. The name is older than any of this: early shutters were fired by squeezing a rubber air bulb on a hose, and the shutter stayed open for as long as the bulb was squeezed.
Not sure you need minutes at all? Long exposure photography covers what a slow shutter does to water, clouds, crowds and lights, and which subjects want which shutter speed.
Why timed shutter speeds stop at 30 seconds
Shutter speeds run in a doubling sequence: 1 second, 2, 4, 8, 15, 30. Each step lets in twice the light of the one before, which is one stop. On most cameras the sequence ends at 30 seconds, and the next click of the dial is not 1 minute but B. A few newer cameras carry timed speeds on past 30 seconds into minutes, and some have a bulb timer that does the same job (more on that below), but the 30 second ceiling is still where most cameras hand the job to you.
The doubling does not stop at the ceiling. One minute is one stop more than 30 seconds, 2 minutes is two stops, 4 minutes three, and so on. Keep that ladder in your head and every bulb exposure becomes a matter of counting steps.
B, T and the bulb timer: three ways to end an exposure
Every long exposure has to start and stop. The difference between the long exposure settings on cameras is only who decides when it stops, and how.
- B (bulb). The shutter opens when you press and closes when you let go. Nothing else is needed, but something has to hold the button down for the whole exposure, and that something should not be your finger.
- T (time). One press opens the shutter and a second press closes it. Not every camera has T, and some name it differently, but where it exists it suits long exposures better than B, because nothing has to stay pressed.
- Bulb timer or intervalometer. You type in a length first, say 4 minutes, press once, and the camera or the remote closes the shutter when the time is up. This is the most accurate of the three, because no one has to watch a clock.
Some cameras also show the picture building up on the screen during a B exposure, so you can stop it when it looks right instead of calculating. Some go further and add only new bright light to the picture, which suits light painting and star trails, because the sky and foreground do not keep getting brighter while the light moves. The camera manual will say whether yours has any of these and where to find them.
Setting up a bulb exposure
B is a shutter time, not a different way of exposing. You still choose the aperture and the ISO, and on most cameras you reach B from manual mode by turning the shutter speed past 30 seconds. On some cameras B has its own position on the mode dial. It is not usually offered in aperture or shutter priority, because in B nothing is automatic.
- Put the camera on something that will not move. A tripod on firm ground, legs spread, the center column down. How to use a tripod properly covers the details that matter more over four minutes than over four seconds, such as weighting it in wind and keeping your feet off a wooden deck.
- Compose and focus while you can still see. Autofocus struggles in the dark and an ND filter blacks out the viewfinder. Focus first, then switch the lens to manual focus so the camera does not hunt when you press the release.
- Turn off image stabilization if the lens or camera manual recommends it on a tripod. Some systems can drift when there is no shake to correct.
- Connect a release. A wired cable release with a lock, a wireless remote, a phone app or an intervalometer. A release with a lock is the cheapest way to turn B into something you do not have to hold.
- Set aperture, ISO and the time you worked out (the next section shows how), then start the exposure and time it with the camera’s elapsed time display, the remote’s timer or a stopwatch.
- Check the result and correct by stops. Look at the histogram, not the screen, which looks brighter than it is at night. Too dark by about a stop? Double the time. Too bright? Halve it.
On a camera with an optical viewfinder, cover the eyepiece during long exposures. Light coming in through the viewfinder can reach the sensor and leave a haze or a glowing patch, and the longer the exposure, the more of it builds up. Many camera straps carry a small eyepiece cover for exactly this.
A self-timer can delay the start of an exposure so the camera settles after you touch it, but it has no say in when a B exposure ends. That is the job of the release, the timer or your second press on T.
Working out the exposure when the meter cannot help
Your camera’s meter still measures the light in B, but it can only suggest a time it is able to set, and its scale runs out at 30 seconds. In a dark scene it often cannot read at all. So the time is calculated, and both of the methods below rest on the same rule: each stop of light you take away doubles the time the shutter has to stay open.
Method one: a fast test frame at high ISO
At night, set the widest aperture and a high ISO, and adjust the time until a test frame looks right. That takes seconds, not minutes. Then count the stops between the test settings and the settings you want to use, and double the test time once for each stop. ISO 6400 to ISO 100 is six stops (6400, 3200, 1600, 800, 400, 200, 100). f/2.8 to f/8 is three (2.8, 4, 5.6, 8).
Say a test frame of 10 seconds at f/2.8 and ISO 6400 looks right. Here is the same exposure at other settings:
| Aperture and ISO | Stops darker than the test | Multiply the 10 s test by | Bulb time |
|---|---|---|---|
| f/2.8, ISO 3200 | 1 | 2 | 20 s (still a timed speed) |
| f/2.8, ISO 1600 | 2 | 4 | 40 s |
| f/4, ISO 1600 | 3 | 8 | 1 min 20 s |
| f/4, ISO 400 | 5 | 32 | 5 min 20 s |
| f/5.6, ISO 200 | 7 | 128 | 21 min 20 s |
| f/8, ISO 100 | 9 | 512 | 85 min 20 s |
The table also shows why few people shoot an 85 minute frame at f/8 and ISO 100 just because the arithmetic allows it. Stars move across the sky the whole time, the sensor warms up, and one passing car can ruin the lot. Pick the settings for the picture you want, then let the doubling tell you the time. If you want stars as points rather than streaks, the exposure has to stay short, and photographing stars and the Milky Way explains how short.
Method two: meter without the filter, then add the filter’s stops
In daylight, B usually comes from a neutral density filter, which darkens the whole scene by a fixed number of stops. Meter and focus without the filter, note the shutter speed, fit the filter, then multiply that speed by 2 once for every stop the filter takes away. A 6 stop filter multiplies by 64. A 10 stop filter multiplies by 1,024. Any result over 30 seconds needs B.
| Metered without a filter | With a 6 stop filter (x64) | With a 10 stop filter (x1,024) |
|---|---|---|
| 1/250 s | about 1/4 s | about 4 s |
| 1/60 s | about 1 s | about 17 s |
| 1/30 s | about 2 s | about 34 s (B) |
| 1/15 s | about 4 s | about 1 min 8 s (B) |
| 1/8 s | 8 s | 2 min 8 s (B) |
| 1/4 s | 16 s | 4 min 16 s (B) |
| 1/2 s | 32 s (B) | 8 min 32 s (B) |
| 1 s | 1 min 4 s (B) | 17 min 4 s (B) |
Read down the right hand column and the pattern is plain: with a 10 stop filter, anything that meters slower than about 1/30 second without it ends up in B. Strong filters also tend to add a color cast and let light leak in around their edges on long exposures, so check the first frame closely. The ND filter exposure calculator does this multiplication for you, for any filter and any starting speed.
Not sure how many stops your filter takes away, or what ND8 or ND1000 means? The ND filter guide converts filter numbers to stops and covers metering before and through the filter.
You can practise the filter arithmetic in the camera simulator. Like most cameras it stops at 30 seconds, so the job is to find an exposure that still fits under the ceiling: fit an ND filter of 6 stops or more and use a shutter of 2 seconds or longer, correctly exposed.
Film is different: reciprocity failure
On a digital sensor, doubling the time doubles the exposure however long the exposure gets. Film does not keep that promise. Past about a second, most films respond less and less to each extra second, so a calculated bulb time of 4 minutes may need 8 or more to look right, and colors can shift too. The amount depends on the film stock.
Shooting film in B? Reciprocity and reciprocity failure explains how much extra time film needs and how to correct a long exposure step by step.
What a long open shutter costs: shake, heat, noise and batteries
Shake. Over several minutes, the jolt of pressing the button or the mirror flipping up lasts a tiny fraction of the exposure and rarely shows. What shows is anything that keeps moving for longer: a finger held on the release, wind on the tripod, footsteps on a pier, a strap flapping. That is why a release matters more for B than any shutter setting. Mirror slap and shutter shock do their damage at much shorter speeds, which camera shake explains along with every other source of blur.
Heat and hot pixels. A sensor that stays switched on for minutes warms up, and warm sensors produce thermal noise: a speckle of bright colored dots, the hot pixels, and sometimes a glow at one edge. Warm nights and long exposures make it worse; cold nights help.
The noise reduction wait. Long exposure noise reduction fixes most of that by taking a second, black exposure of the same length with the shutter closed, and subtracting its hot pixels from your picture. The cost is time: a 4 minute bulb exposure ties the camera up for about 8 minutes. That is a fair trade for one carefully planned frame and a poor one at a fireworks show. Long exposure noise reduction (LENR) covers when to leave it on.
Seeing red, green and blue specks in the dark parts of a long exposure? Hot, stuck and dead pixels shows how to tell them apart and get rid of them.
Batteries. The camera draws power for the whole exposure and again for any noise reduction frame, and batteries lose capacity in the cold. Start a night of B work with a full battery and keep a spare in an inside pocket. Camera batteries has more on cold weather.
What photographers use B for
- Fireworks. Open the shutter as a shell goes up and close it after it bursts, or keep it open across several bursts. Some photographers hold a black card over the lens between bursts to collect several in one frame without the sky brightening. How to photograph fireworks has the settings.
- Lightning. At night, open on B and wait. When a strike lands in the frame, close the shutter. How to photograph lightning covers the settings and, more importantly, safety.
- Light painting. You walk through the frame with a flashlight or light tube for as long as the drawing takes, which is rarely a neat 30 seconds.
- Light trails and star trails. One long exposure can hold minutes of traffic or sky. Many photographers now shoot a run of shorter frames with an intervalometer and stack them instead, which keeps noise down and survives one bad frame. Light trail photography compares the two.
- Daytime ND work. Smoothing water, streaking clouds and emptying busy squares, as in the filter table above.
- Night landscapes by moonlight. Low ISO and a small aperture for a clean, sharp frame, at the cost of minutes per exposure. See how to photograph landscapes at night.
Some photographers built whole bodies of work on the open shutter. Hiroshi Sugimoto opened his shutter when a film began in a cinema and closed it when the film ended, about two hours later; the screen came out as a blank white rectangle whose light revealed the auditorium. Michael Kenna made black and white landscapes at night and dawn with exposures from about a second to as long as ten hours. Both are lessons in deciding what an exposure that long is for before you make one.
Want the stars to sweep in long arcs around the pole? Star trail photography covers single exposures, stacking and composition, and the star trails calculator tells you how long the arcs will be.
Common mistakes and misconceptions about bulb mode
- “Bulb is a mode that works the exposure out for me.” It is the opposite. B is the one shutter setting where the camera decides nothing about the time. Aperture, ISO and duration are all yours.
- “The meter will tell me when to stop.” The meter cannot suggest a time past its own scale, usually 30 seconds. Use a test frame or filter arithmetic, then check the histogram.
- “I can hold the button with my finger.” For a few seconds, perhaps. For minutes, your hand keeps the camera moving the whole time. Use a locking release, T or a timer.
- “The self-timer will handle it.” A self-timer only delays the start. Something still has to end the exposure.
- “Longer is always better.” A longer exposure collects more light, but also more sky glow from light pollution, more star movement, more heat noise and more chances for something to move. Choose the time the picture needs.
- “My calculation was wrong, the frame is too dark.” Check the stops before blaming the arithmetic. A common slip is counting a 10 stop filter as x10 or x100 when each stop doubles the time: 2 to the power of 10 is 1,024, which is why a 10 stop filter is also sold as ND1000. Another is forgetting that the light itself is fading at dusk while you calculate, so a time worked out at sunset is too short ten minutes later.
- Leaving the eyepiece open, autofocus on or the noise reduction wait unplanned. Each one is small, and each one has cost photographers a whole night’s frame.
Try this: a calculated bulb exposure in ten minutes
Go out about half an hour after sunset, or into a dim room at night with one lamp on. Put the camera on a tripod, focus, and switch to manual focus.
- Set the widest aperture and ISO 6400. Shoot test frames, changing only the time, until one looks right on the histogram. Write the settings down.
- Choose your working settings: ISO 100 and an aperture two stops smaller than the widest (for example f/5.6 if the widest is f/2.8).
- Count the stops: six for the ISO, two for the aperture, eight in all. Double the test time eight times, which is multiplying by 256. A 2 second test becomes 512 seconds, 8 minutes 32 seconds.
- Switch to B, start the exposure with a release or a bulb timer, and time it.
- Compare the two histograms.
How to judge it: if the bulb frame’s histogram sits within about a third of a stop of the test frame’s, your arithmetic and your timing were right. If it is darker, the light was fading or you lost count; if it is brighter, a light came on or you held too long. Then look at the two frames at 100 percent. The ISO 100 bulb frame should be visibly cleaner, and that difference is the reason to go to all this trouble. If the light was fading too fast to make the comparison fair, do it again indoors with the lamp.
Want to check you could work out a night exposure on the spot? The long exposure and night photography quiz asks the questions this page answers, and explains each answer.
Questions about bulb mode
What is the longest exposure bulb mode can make?
In principle, as long as the shutter is held open; in practice the battery, sensor heat and sometimes a limit set by the camera decide. Some cameras cap a single B exposure, so check the manual before planning one of several hours.
Why can I not find B on my camera?
On most cameras B appears only in manual mode, after the 30 second setting. Some put it on the mode dial instead, and some simpler cameras and phone camera apps do not have it at all. Phones usually offer a long shutter or night mode that blends many short frames, which phone long exposure explains.
Is bulb mode the same as time mode?
No. In B the shutter stays open only while the release is held. In T one press opens it and a second press closes it, so nothing has to be held. Both give you any time you like; T is simply easier on long exposures.
Can I use bulb mode in daylight?
Yes, with a strong ND filter. Without one, even the smallest aperture and lowest ISO will usually overexpose a daylight scene in far less than a second. With a 10 stop filter, a scene that meters at 1/8 second needs 2 minutes 8 seconds.
Do I need bulb mode for star trails?
Not necessarily. You can make one exposure of 30 minutes or more in B, or shoot many 30 second frames with an intervalometer and stack them. Stacking keeps the sensor cooler and lets you discard a frame spoiled by headlights or an aircraft.
Does the camera meter in bulb mode?
The meter still reads the light, and some cameras show how far over or under it thinks the scene is, but it cannot suggest a time past its scale and cannot read very dark scenes. Treat it as a hint and work the time out with a test frame or the filter factor.