I hope it's OK to post this. Don't mean to hijack the thread or anything like that. OK, so if the OP or others would like to try the zoom effect, here you go:
Equipment:
- A VERY sturdy tripod

- 3 stop ND filter
- Camera and wide angle lens
- Ability to bulb and keep the shutter "locked": I have a remote release that can be locked
- A stop watch or a watch you can see in the dark
- A decent lens that won't wobble when you zoom it in and out
Method:
- Stop down the lens to F/8
- Set the camera to bulb
- Put the ND filter on the lens
- First of all take a test shot without zooming to get the exposure and timing right. For me it was 207 seconds.
- Next, set the lens to the widest angle: This is important because if you set it to anything greater than the widest setting then as you zoom out, you may cross your initial "burn threshold" and so the zoom effect will start going inwards. Read on further below.
- When you lock the shutter you are going to do five things, four of which you will repeat:
1) Establish your burn in - I call this the "burn threshold" because basically you are going to burn in the buildings and the outline of the London Eye BEFORE you start the zooming. If you started zooming from the offset then everything would be blurry. Make sense? For this, I waited about 1 minute.
2) Zoom in to the extent of the lens
3) Wait 1 second
4) Zoom out to the extent of the lens
5) Wait 1 second
Then rinse and repeat from 2 to 5. Just keep going until you reach your duration.
So a running commentary would be:
Zoom out. Trip the shutter and lock the button on the remote release. Start the stop watch or check the time and seconds. Wait for 1 minute to burn in the background. Zoom the lens VERY slowly to the end. Wait 1 second. Zoom out the lens to the extent. Wait 1 second. Repeat the zoom/wait until you reach your duration. Close the shutter and chimp
The "wait 1 second" gives you that outer ring and inner ring.
If you don't use an ND grad then the exposure time will be quicker and you won't get as smooth a streak. It will be kind of jagged. If that makes sense. Plus it reduces the ghosting as you zoom.
Enjoy and have fun experimenting!