Consider that the camera is in shutter priority and the exposure time is set to bulb mode. Now the camera doesn't know how long the shutter is going to be open in advance and so how does the camera ...
My camera has a 'bulb mode,' and several questions on photo-SE refer to using Bulb Mode. A few questions: What is 'Bulb Mode?' Why is it called 'Bulb Mode?' What is 'Bulb Mode used for?