Bulb mode/setting (B) is a shutter speed option that you can select. It allows your shutter speed to be any length you choose: one second, one minute, 10minutes, etc. It often used to achieve the right exposure for low-light situations like night photography. The Bulb setting usually requires that you hold down the camera’s shutter release button during the entire exposure, or sometimes press it twice. The Bulb name comes from the detachable rubber ‘bulb’ pneumatic shutter releases that were used with early cameras.