✓Professional timelapse photography with Bulb Ramping from t-moco ...
Up to 6 bulb shutter stops with smooth exposure gradients …
Example: A 6-stop sunset bulb ramp (1/8 sec [125ms] to 8 sec) at an interval of 6 minutes results in a smooth and continuous 6-stop exposure increase during 36 minutes of reduced ambient lighting. If more stops are needed, you can add up to 6 ISO steps.
Bulb Ramping
Description
Adjusting shutter speed dynamically as the ambient lighting changes (eg. at sunset or sunrise) is called exposure 'ramping'