FAQ: Bramp Shutterspeed Limitations

The fastest shutter speed available in Bulb mode is controlled by the speed of the camera's circuits. The fastest shutter speed is the inverse of the shutter close lag – so a 40ms close lag gives 1/25 fastest speed. Think of it this way – the fastest speed available is the one you get if you were to open the shutter, and as soon as it opens, IMMEDIATELY signal it to close. It won’t close any faster than the shutter close lag time – hence the logic. Promote Control also adds a bit of a safety measure, which can be disabled by setting “Shutter close lag safety margin” to “Minimal”.
The formula is: ( 1/Shutter Close Lag) rounded next slower one third stop