CHAPTER 6 HOW TECHNOLOGY LETS THERE BE LIGHT^91
What is needed in this situation is a relatively long flash with reduced brightness. But flash units are pretty much full-speed ahead
until it’s time, according to the thyrister, to turn the light off. Short exposure times call for focal plane sync, or FP sync
(also called HSS, for high speed sync). With this setting, the flash executes a series of short bursts at a high frequency, about
50MHz. None of the bursts swell to the flash’s full brilliance, but the series lasts long enough to illuminate the full frame evenly.
FP sync can work with some cameras at shutter speeds up to 1/8,000 of a second.3
Photographers use another type of synchronization, slow
sync, to capture movement using flash. The flash is timed to
go off only when the shutter is fully open. Meanwhile, with
most of the shutter open, the image sensor is already
recording the moving subject of the photo. Because the sub-
ject is moving and because the scene is dark enough to
require a flash, this part of the exposure is more suggestion
than documentation. The subject is blurred, and even the
blur is hard to distinguish except for any lights or bright
reflections. Right before the shutter snaps shut, the flash
goes off, adding a clear, exposed view of the subject. In
this case, the streaks from a car’s lights, captured before the
flash went off, provide the sense of movement.4
But notice how the proper synchronization is crucial to the
effect. If the flash goes off just as the shutter has opened, as
in this photo, the hammer appears to be streaking back-
ward. Slow sync is also called rear curtain sync.5