For starters, adding flash-made motion blur requires a balance between the strobe and some existing amount of ambient light. I find fairly bright ambience in the background works best and gets better still if it silhouettes the subject or if the subject can be positioned such that the bright background light isn’t also illuminating their face. In short, I want a silhouetted subject against a fairly light ambient background.
Next, I dial in the background exposure to get it just right. This might be a bit over- or underexposed—however I see fit. Adjust the shutter speed and aperture independently to create whatever type of ambient exposure you like. And remember that it’s okay if the subject herself is underexposed—preferable even—because you’re going to light her up with the strobe in the next step.
Because we can control strobe illumination independent of ambience, we can effectively make a double-exposure in a single frame. Adding more power to the strobe’s output or moving it closer will make the strobe portion of the scene brighter, without impacting the ambient exposure. Likewise, adjusting the shutter speed to lighten or darken the ambience will have no affect on the strobe exposure. It’s in this discrepancy that the creativity awaits.
Try to use a relatively slow shutter speed when establishing the ambient exposure. Something in the neighborhood of ¼ of a second may be necessary if you’re using a particularly wide lens, but a normal or telephoto lens will often work well with 1/15th of a second shutter speed. The telephoto lens will amplify blur—which will become crucial in a moment.
With the ambient exposure dialed in and a shutter speed that’s technically too slow to handhold, you’re ready to add the strobe. You can position your camera on a tripod if it’s helpful for composition and light testing, but when it’s time to shoot be sure to handhold. The handheld camera combines with the slow shutter speed to introduce the blur in the background and at the subject’s edges, while the strobe’s short duration will effectively freeze the subject as a faster shutter speed would. Combined, they create a tack-sharp subject while the edges and background are filled with motion blur.
I find one of the most appealing aspects of this approach is when it creates a dark outline around the subject’s edge. This enhances the feeling of depth in an image and, along with the bit of blur, keeps the exposure from appearing too static and plain. This edge is a function of the slight change in position between the subject when silhouetted for the ambient exposure and where the subject is positioned when the flash fires. Too much edge blur is typically a product of too much ambient impacting the subject herself.
The amount of blur can be dialed in by changing shutter speed and shutter speed alone. Aperture changes or light position and output changes have no impact. Sure, they’ll impact the strobe exposure, but they won’t affect the level of blur or background illumination. Experimentation is key, and it’s part of the fun. Try faster and slower shutter speeds, paired with more or less exaggerated camera movements during the moment of exposure. In short order, you’ll strike the perfect balance where a hint of motion in the background adds enough blur to be interesting without obliterating essential image-forming detail.