It’s a classic photo technique: slow the shutter speed down in order to convey a sense of movement via the introduction of motion blur. Ed wrote about it here. The photo above of a passing motorcyclist was taken at 1/25 of a second, a speed typically too slow to freeze action… which is, of course, precisely the point.
Typically, shooting between 1/25 of a second and one second should be plenty slow enough to create some motion blur, depending on lighting conditions and exactly how much blur you’re interested in introducing. The ferris wheel photo above was taken handheld, though with my hands planted on top of my car roof, for a full 4 seconds. You’ll notice the side-to-side wavering motion that rendered the shape of the spinning wheel non-circular. To have achieved a cleaner image, a tripod and shutter release mechanism (mechanical or infra-red) would have been necessary. The fact that the wheel’s spin is not perfect works better here in my opinion as it adds to the overall illusory effect.
This New York City cab was taken at 1/15 of a second, which was enough to eliminate all surrounding details and again, to create the feeling of movement.
If the amount of light is too great, thus forcing your metering toward a faster shutter speed, it is possible to use a neutral density filter, which is one of the only filters (besides a polarizer) I still carry. In fact, I rarely even use UV or skylight filters as basic lens protection these days. The neutral density filter cuts the amount of light hitting the sensor, thus allowing for slower shutter speeds. Such filters are the classic tool of the landscape photographer attempting to capture moving water in a slower, flowing manner. Polarizers can also have this effect while cutting reflections and altering the look of the sky.
Finally, panning your camera can help emphasize motion by blurring the background but freezing the main subject.
Posted by Rey