This is a technique I learned years ago in photography school and have been having fun with lately. Hope you have as much fun playing with it as I do!
Zooming during a long exposure tips
Works well on night scenes and ones with many light sources
Set up your camera on a tripod with a remote trigger or release
Set your camera to Manual mode
Set your focus by zooming in all the way, then lock the focus (using either back button focus or focus using Auto-focus and then switch it off to Manual focus)
Zoom out so you can see the whole image as you want it framed
Then start your exposure and zoom your lens out while the shutter is opened (you may need to practice a few times to make it go smoothly)
Experiment with different zooming speeds and timing, as example: if you exposure is 5 seconds do one with 3-4 seconds at the beginning, then zoom at the end; then do another when you zoom immediately and do most of the exposure at the end
Exposure must be 2 seconds or longer to make this work, otherwise it becomes really tricky to zoom
If it is too bright to get a 2 second exposure, make sure your ISO is as low as your camera goes and your aperture is set to your smallest setting (f/22 or f/32). If it is still too bright you may need to use a Neutral density filter to cut down the amount of light coming through the lens
Here are a few examples done using this technique.
I encourage you to experiment and play around with technique and other similar variations. Share your images in the comments below and if you have other tips please tell us!