Bulb mode is found on most cameras when you scroll to your slowest shutter speed in manual mode or shutter speed priority – you will probably reach 30″ (seconds) and then scroll one more and you will hit Bulb or “B”.
Why use Bulb?
Bulb mode simply allows you to take an image for as long as the shutter is open. The function is to allow light in for the duration you hold your finger on the shutter release button, until you release your finger, then the shutter closes. You usually have a timer on the back of the camera indicating the duration the shutter is open. But it only really makes sense to use it on longer exposures.
It would be fun to suggest that you have to stand with your finger on the shutter button when taking an image. This is not necessary anymore (more on this later) as you can download apps or buy a cable release for your specific camera and ‘lock it’ open for the desired time while on bulb mode. This ensures you have no camera shake. The slightest movement while the image is being recorded, even walking around the tripod could create camera shake.
Another tip is to use mirror lockup (tool buried deep in your menu mode) to avoid any camera shake when the shutter closes at the end of an image.
Where did the term Bulb come from?
Back in the day there was a pneumatic valve bulb that was used as a cable release. It looked like a pump. When the bulb was pressed, it would send a blast of air from the bulb until the photographer released the shutter and then it would close… but this meant that the bulb had to be held by the photographer for as long as the shutter would need to be open. Painful if doing 20 min or one hour long exposures!
Why would you want to do these extremely long exposures? Usually, when you are taking pictures of stars. if you are in a pitch black countryside with no extra lights, you would need your tripod, bulb mode and a very long exposure probably starting at 20 minutes and checking out the result and possibly trying again!
Long exposures of 10 seconds or more could be used for a technique called light painting, fireworks or lightning for example.