What is door B in SLR photography?

When looking at the parameters of digital SLR camera … you can often see a B door written behind the shutter speed index …

Excuse me ... what does this door b mean? A: Door B is a gear of the shutter. When the shutter release button is pressed with the door B, the shutter opens (exposure starts). When the shutter release button is lifted and released, the shutter is closed.

... what role does it play in digital SLR cameras? -Answer: It is to gain a longer time (longer than 1 sec. Theoretically, as long as the battery has electricity to supply power to the shutter, door B can be opened indefinitely; As long as there is nothing wrong with the mechanical shutter, the shutter can be opened indefinitely.

... generally speaking, consumer digital cameras ... is there this door B? A: Generally, most people don't have Gate B. ..

(T-door is rarely installed on cameras in recent years, only on a few models, mostly professional machines)

Generally, door B should be used together with tripod and cable release (tripod and cable release can reduce the camera shake and the vibration when the shutter itself is opened and closed, so as to obtain a clearer picture). Generally, door B is mostly used for night photography.