Want to photograph a moving subject so that it’s not blurred and yet preserves a clear sense of motion, rather than seeming frozen in space? Try stroboscopic flash. Like high-speed flash syncing and wireless TTL flash, it’s one of those seemingly byzantine flash techniques that most photographers never get around to mastering. Here’s what you need to know.
Stroboscopic flash uses multiple flash pops to capture a moving subject relatively sharply and repeatedly as it travels across a single frame. A feature of many high-end flash units, stroboscopic flash requires working at night or under very low ambient light.
Backdrops should be simple, uncluttered, and dark. So, to ensure adequate separation of subject from background, pick a light-toned subject. You’ll probably want space between subject and backdrop, as well — if they’re too close, the flash might hit the backdrop, bumping down the contrast and visual impact.
Strobo flash relies heavily on trial-and-error to find the best mix of camera and flash settings, as well as subject speed. For that reason, it’s easiest if you’re shooting with a DSLR (and not a film camera), because its LCD will show you instantly if your settings are going in the right direction.
Start in your camera’s manual-focus and exposure modes, and preset the focus and a shutter speed long enough to include the number of flash pops you want to record. Then fine-tune overall exposure by adjusting the aperture and ISO settings accordingly. Shutter speeds are typically ¼ sec or longer, so to keep the subject relatively sharp, use a tripod.
For flash control, you have two main considerations: flash frequency (a.k.a. pops per second or Hz) and power (full, 1/2, 1/4, etc.). The faster your subject, the higher a flash frequency you will need. For moderately fast subjects, start at 8 to 15 pops per second. For fast-moving athletics, try between 15- and 40Hz. Anything higher, and the individual images smear into a blur. To capture the trajectory of the paper airplane, above, Lars Hagberg used 10 pops across a 1/3 sec exposure (or 30Hz).
The more pops, the dimmer each will be, so for higher frequencies, boost your exposure by setting a higher ISO, larger aperture, or moving the flash and subject closer together.
Stroboscopic flash is a huge battery drainer, so use fresh cells, and wait 15 to 30 seconds between pops to give the batteries time to completely recycle.
Your biggest challenge? Finding a fresh subject that benefits from strobo flash. Forget such clichés as golf swings or pirouetting ballerinas. Someone breaking a rack of pool balls (shot from above), a diver performing jackknives into a pool at night, or a kid going crazy above a trampoline all could make amazing images.
Here are a few common problems you may run into, along with an array of ways to overcome them.
Subject is too light (overexposed):
Set a smaller aperture and/or slower ISO.
Dial down the flash power from, say, 1/8 to 1/16 power.
Move the flash and subject further apart.
Dim the ambient lighting or find a darker location.
Subject is too dark (underexposed):
Set a larger aperture and/or higher ISO.
Set a higher flash power.
Move your flash closer to the subject.
Shoot under brighter ambient light.
Subject is too blurred:
Reduce the flash frequency (Hz).
Slow the subject’s motion.
Set a faster shutter speed.
Dim the ambient light levels.
Background is too bright:
Set a faster shutter speed, lower ISO, or smaller aperture.
Move your subject away from the background.
Shoot under dimmer ambient light.