Let’s say you’re standing on one side of the street and someone is riding a bicycle down the other side. If you use a shutter speed of, say, 1/500th of a second to make your exposure as that cyclist rides past, that rider and the bicycle are both going to appear frozen in place and, assuming you focused accurately, tack-sharp. But if you drop your shutter speed to, say, 1/15th of a second and hold your camera steady, that cyclist is going to be a big blur. That’s not especially helpful. But it’s laying the groundwork for something special. (And, in some cases, it could be exactly the look you need: a tack-sharp scene and a fast-moving cyclist riding through.)
The next step is to keep that slow shutter speed (sometimes even slower, like 1/8th or even 1/4th, is helpful), but move the camera along with the subject’s motion. Panning is the side-to-side movement of the camera. (Up and down movement is called tilting.) So, as the cyclist moves from right to left, you keep the cyclist centered in your frame, all the while panning the camera along with their movement from right to left. When the bike is in the optimum position—usually right in front of you where the movement of its image across the sensor would be fastest—click the shutter. The combination of the slow shutter speed and laterally moving camera will make the bicyclist fairly sharp (although moving legs and spinning wheels will have some blur) while the background will be horizontally blurred. This blur effect not only helps to simplify the background, but it also enhances the feeling of motion, and in many ways, it can make the cyclist look like they’re going fast.
It’s a fairly simple technique to master, with the primary challenge being matching the speed of the motion with the appropriate shutter speed to provide enough blur to create the feeling of motion without blurring the parts of the subject you want to remain sharp.