Firstly you'll obviously need a zoom lens in order to do this. Exposure length needs to be long enough to be able to physically move the lens. The shots I took below (at night) were at 0.8 or 0.6 seconds respectively; if you're shooting during the day you'll need an ND filter to reduce the light and allow you to expose for longer. You'll need to focus on the subject first (using back-button focusing is helpful), then press the shutter and then move the lens - either in or out - quickly but smoothly (the smoother the better). It takes practise and the shots might look jerky or strange. Whether you like the result will be a matter of taste and the effect won't work with many subjects. These two images were my favourite ones of Rome's Colisseum, where I tried out this technique a couple of years ago.
I was using a wide-angle (10-22mm) lens on my old Canon 400D and shot at f/4, ISO 400. I did have to do a little cloning on the first image to remove an ugly light trail that just didn't look right in the shot - this is a hazard of the method. The Colisseum is an often-photographed building and photographs can sometimes look a bit dull, like the one below. I think the magnificent structure lends itself to being zoomed, given all those wonderful arches!
Coming Soon - Breaking the Photography Rules Part 6 - Shoot Out of Focus
See also: Part 1 - Go Out in the Midday Sun
Part 2 - Ignore the Thirds Rule of Composition
Part 3 - Turn it Upside-Down!
Part 4 - Shooting Landscapes in Portrait Orientation
Part 6 - Shooting Out of Focus
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