Syncing Up

FLASH SYNC is simple in principle: Flash must fire when the shutter is open to illuminate the scene. But two factors make it more complex. One is electronic flash, whose burst is no longer than 1/1000 sec, and as short as 1/100,000 sec. So sync timing must be very precise. Another is the focal-plane shutter, used universally on film and digital SLRs. When these shutters fire, the first of two curtains slides open to uncover the film or the sensor. At the end of the set shutter interval, the second curtain closes. For flash exposure, the camerasync circuitry triggers the flash at the moment the first curtain is fully open. But focal-plane shutters can fire only so fast. To produce faster shutter speeds, the gap between the curtains is nanowed so that only a slit travels across the imager. If the flash fires during this exposure, one of two things happens: Only a band of the film gets exposed by the flash (older cameras), or you donget a flash exposure at all (newer cameras). Hesharp, yet blurred: A brief flash burst froze the motion during a slow 3/80 sec shutter speed, for a classic flash ghost. The fastest shutter speed that will expose the full frame to the flash on todayDSLRs is 1/180 sec to 1/250 sec, depending on model. Not very fast, and if there is movement in the frame, the ambient exposure can blur around the frozen flash exposure, a phenomenon called ghosting. But wait! Most high-end accessory flashes for current and recent SLRs provide highspeed flash sync, which works by firing stroboscopic pulses as the shutter slit travels across the imager. Neat trick, and you can get flash sync at any speed —even 1/8000 sec. But maximum flash reach is seriously limited, as the full flash power is distributed over a lot of little pops. So the best use for high-speed sync may be for fill in outdoor portraiture. This way, you can shoot wide open with that f/1.4 lens without a pile of ND filters, even in bright daylight.

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