What is Bulb?
Bulb is a shooting mode that is over one hundred years old. Most DSLR cameras have the slowest shutter speed of 30 seconds (30”), if you want it longer than 30 seconds you use Bulb. So Bulb is a setting that lets you keep the shutter open for as long as you keep the shutter button depressed. Bulb mode is named after the bulb on detachable rubber pneumatic shutter releases that came with early cameras. The pneumatic release kept the shutter open for as long as the photographer squeezed the bulb, hence a “Bulb” exposure.
When would you use it?
Astrophotographers shooting the night skies use bulb mode to capture light from distant stars. Landscape photographers use bulb, together with a Neutral-Density (ND) filter when shooting coastal landscapes. The long exposure makes the surface of the sea look smooth and ethereal.
Basically, if your exposure is over 30 seconds you need to use Bulb mode and have your camera on a tripod. The tripod needs to be a very strong and stable one because the camera can’t move while the shutter is open. If it does move the image will be soft and blurry.