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I've been asked a few time to explain why there is a vortex in many of the "blurry" photos I've taken from cars. The above diagram I hope helps explain this.

Assume I'm holding the blue camera in my car in the left most image (my wife is driving, not me). I see the interesting patch of stars on the side of the road and center my camera's viewfinder on the yellow star. With a the shutter speed set at 1/5s, I press the shutter release and rotate the camera so that it stays centered on the yellow star. At the end of the 1/5s, the camera is now pointed as in the middle diagram. You can now see that as the viewpoint has changed, the other stars around the yellow star appear to have moved in a counter clockwise direction around the yellow star. Since the shutter is open the whole time, you'll see a circular blur around the center star.

Using trigonometry, you can calculate just how many degrees you need to rotate the camera to keep a point at a given distance at a particular speed fixed in the center of the image. As should be obvious, the faster you go, the faster you need to rotate the camera, and the closer to the vehicle, the faster you need to rotate the camera.

Holding the camera steady pointed 90 degrees fromt the direction of travel while taking the photo is the equivalent of fixing the camera on a point at infinity, so you'll get straight lines of blur across the image. If you move the camera much faster than required to keep a point in the image in the center, you'll get lines of blur representing only the left half of the vortex. These lines can become a near vertical blur in the image. And, although I haven't tried this yet intentionally, if you rotate the camera in the direction of travel, you should get only the left side of the vortex.

If you are really good and fast (or can use longer shutter speeds), you should be able to get multiple vortexes by varying the panning speed and direction. I see hints of this in many of my photos, but I can't say I really tried to do this intentionally yet.

A few hints to make it easier for you to get good results. 1. Use manual focus, if you wait for the auto focus to focus you'll miss the shot. 2. Start panning before you hit the shutter release. With an SLR, you won't see anything in the view finder when the picture is being taken, so you'll need to just continue panning at the same speed. 3. Find someone else to drive.

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