A camera setting seen on old SLR (single lens reflex) models that allows the shutter of the camera to be left open for an extended period of time. Modern digital DSLR cameras have a similar function, also called a "B setting". For astrophotography and CCD imaging the setting is crucial in order to enable long-exposures, where the shutter is left open for several minutes or even hours in order to capture enough light from a deep sky object.