Here’s a neat video that shows what you get in a star trails time-lapse if you slowly move your lens’ zoom ring over the course of the long exposures. Long exposures naturally create star trails around Earth’s axis of rotation, but throw in some motorized zooms and you can turn those ordinary trails into some pretty trippy effects.
This particular project used the ZenFocus motorized follow-focus/zoom system, which helps you program movements to your focus or zoom ring using a belt that’s compatible with any lens.
Here’s what a single stacked long exposure photograph looks like with ordinary star trails:
Do this across sequences of shots, and you can create a beautiful time-lapse in which stars draw trails across the sky (you can see this in the video above):
Start involving your zoom ring, and things get interesting. If you slowly zoom over the duration of the exposures, you’ll find that the stars begin to create spirals (rather than circles) around Earth’s poles.
Here’s what you get if you slowly zoom a Canon 17-40mm f/4 lens over the course of a 1 hour total exposure:
This next one shows a zoom from 40mm to 17mm over 2 hours of exposures (these are stacked images created by combining shorter exposures):
The ZenFocus system costs €305 (~$360) over in the ZenSlider online store, but you can use any motorized, programmable zoom system for this type of effect.