I am not sure if there is a threshold value but I do know that on my digital there is no such thing as a Bulb setting. I think the longest setting I can get is 1/60. I guess I will have to fire up my Voitlander and try some low light shots with it. BTY how do you know, when using the bulb setting, how long to keep the lens open? Is it just a trial and error thing or what.