When many photographers think about techniques for accentuating motion blur, they consider shutter speed exclusively. And for good reason—as a stationary camera and a moving subject create motion blur, so does panning the camera to keep the subject sharper while the background blurs. But the problem with this approach is that it’s awfully easy for the entire scene to become too blurry. Combine motion with a flash of light, however, and you can create an ideal blend of a sharp subject with a trail of motion blur.
It works because the strobe of light from the flash is such a short duration—1/1000th of a second, for instance—that it’s effectively acting as a shutter speed of 1/1000th for the instant it fires, briefly catching the subject in illumination from the strobe. Meanwhile, ambient light registering on the moving subject for the duration of the exposure will create motion blur. It’s a lighting technique that provides the best of both worlds: sharpness where you need it combined with blur to add to the illusion of motion in a still photograph.
To make this kind of image, all that’s needed is an on-camera flash or external strobe. You’ll want to think about the exposure in two separate parts—the ambient and the strobe. Step one is to determine the correct ambient exposure, which should likely be a bit underexposed since it’s strictly to provide motion blur. Too much ambient exposure will create an overpowering blur. It ultimately takes experimentation in any given scenario to determine what looks best. Remember that there’s no single right amount of ambiance or blur; if it looks good to you, that’s what counts.
Let’s say, for example, that the correct amount of ambient exposure is 1/60th of a second at ƒ/8 and ISO 200. Depending on how fast the subject is moving, this may or may not provide enough exposure duration to create motion blur. So before proceeding to the strobe exposure, make sure the ambient creates not just adequate illumination but also motion blur that’s to your liking. This will likely entail slowing down the shutter speed to, say, 1/30th or ¼ or a full second or more—and then adjusting the aperture and ISO as necessary to maintain the correct exposure for the ambient light. If what you’re photographing is very fast moving, a faster shutter speed may be slow enough to allow for blur.
With the ambient exposure and motion blur dialed in, it’s time to add the flash. With a speedlight, on-camera flash or smart external strobe, TTL may be available to determine the correct amount of flash for a given scene. Short of that, however, or if you’d like to really understand exactly how this exposure technique works—adjust your strobe’s output manually. Set it to quarter power to start because it provides a couple of stops of adjustability in each direction, then shoot a test shot to see if it’s correct. If you need more flash, turn up the power to half or full power and shoot another test. If you need less, turn down the flash output to 1/8th or less.
If these output adjustments alone don’t hit the perfect flash exposure, consider moving the flash closer to the subject to make it brighter or farther away to bring it down. Failing that, you’ll have to adjust the aperture. This is the last resort because you’ve already got the ambient exposure and the blur dialed in, so changing the aperture will entail adjusting the shutter speed by an equal amount to compensate.
With the ambient and flash exposures now dialed in and working together, you’ll see a flash exposure that renders the subject tack sharp along with an ambient exposure to provide the blur. But still, there’s one more setting to consider. It’s determining whether the strobe should fire at the beginning of the exposure or at the end.
Typically, when the shutter curtain opens up to allow light into the camera, the flash fires immediately. That’s called “first curtain” flash sync. This is fine under normal circumstances, but when the subject is moving and the exposure is set to register that movement in the photograph, there comes a problem. When the flash fires first, any movement that occurs after that burst of light puts the motion blur in front of the sharp subject. And that just doesn’t look right.
Look at the shot of the mountain bikers above. This was made with first curtain flash sync because the strobe fired and rendered each rider sharp and the subsequent movement put their motion blur—especially from their lights—moving forward from the bikes. But our brains think that blur should be following the subject. To make that happen in pictures, the flash has to fire after the ambient exposure is recorded. This is called second curtain sync.
Second curtain flash sync (sometimes called rear curtain or slow sync) means that the shutter opens for the duration of the exposure and then just before closing—before the second curtain comes down—the flash fires. In practice, this puts the motion blur from the ambient exposure behind the moving subject where it looks the way we think motion blur should generally look. The shot of the playing card in motion shows a more natural blur effect as it appears the hand and card have moved quickly down into the frame, with the blur trailing the movement thanks to second curtain sync.
Second curtain flash sync is available in many digital cameras, speedlights and external flashes, and it’s typically as simple as changing one menu setting to tell the camera when the flash should fire. But when that flash fires can mean the difference between perfect motion blur and something that doesn’t quite make visual sense.