Answer: Because bulb exposure times can be set with a precision of 0.001 seconds. The precise, tiny increments allow smooth ramping of exposure when light is changing, i.e. at sunset or sunrise.
When you run GBTimelapse through a laptop connected to a camera, GBTimelapse uses a software timer to control the camera's bulb exposure times. Although the software timer can be set as short as 0.100 seconds, it is most accurate and repeatable at times of 0.500 seconds and longer. The accuracy at a shorter duration is limited because of Microsoft Windows, which is not a "real time" operating system.
New FeatureNow, GBTimelapse (Version 3.7) can use the MX2 Dolly Engine from Dynamic Perception to control bulb exposure times. Because the MX2 runs in "real time" it has a much more accurate timer not limited by Microsoft Windows. This allow you to use much shorter bulb times and still avoid flicker.
How-to and setup tipsOn a 5D Mk II, being able to
To use the MX2, connect both the camera and MX2 to your laptop with USB cables. Then connect the MX2 "camera" port to the camera with a shutter cable and the "ext 1" port to the camera flash port with a PC/flash sync cable.
For bulb control you can run the MX2 with power from the USB cable. To do bulb control and motion control, you will need to power the MX2 with a separate battery.
For a demo showing how to set this up, please watch this video...
... and another demo showing how to use the MX2 for a sunset time-lapse...
Minimum accurate bulb durations for some camera models
|Model||MX2 Bulb Timer||Software Bulb Timer|
|5D Mk III||0.040 seconds||0.300 seconds|
|5D Mk II||0.033 seconds||0.300 seconds|
|7D||0.033 seconds||0.500 seconds|
|40D||0.035 seconds||0.400 seconds|
|T3i||0.035 seconds||0.500 seconds|
|XSi||0.047 seconds||0.400 seconds|