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A light sensitive sensor at each remote flash, which triggers the flash in sync when it sees the flash of another manual flash unit.

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Optical Slave Trigger (1965)


Wireless sync

Some of the synchronization methods employ optical or radio triggering that requires no electrical connection to the camera or main flash unit. This allows the camera to move without the restriction of cables. Optical triggering requires at least one flash electrically connected to the camera. A sensor, either built-in or external to a remote slave flash unit, will sense the light from the master flash and cause a remote flash to fire.

One of the problems with optical triggering is that in modern digital cameras a built-in or shoe mount flash releases one or more ‘pre-flashes’. Many optical slave units will respond to the pre-flash, thus firing the slave flash too early. Sometimes this can be prevented by setting the camera to manual (‘M’). However, a good number of cameras will still fire pre-flashes even on a manual setting. This is equally true for compact cameras as well as the more professional digital SLR cameras. Still, a flash connected to the PC jack on a camera or in the hotshoe will usually not fire pre-flashes in the ‘M’ setting and therefore can be used to optically trigger a number of slave flashes.

Optical slave trigger – A light sensitive sensor at each remote flash, which triggers the flash in sync when it sees the flash of another manual flash unit. This light path can be reflected from close walls or the photo background, but light is line of sight, normally blocked by obstacles. Slaves have the advantage of being triggered by the full working value of the final flash power, but bright sun can interfere with them, obstacles can block them, and another photographer’s flash can trigger your lights. Relatively short range, but is a very popular choice in the studio, where few issues exist.

An optical slave is essentially a fast-acting, light-operated switch. It includes a photocell with a wide acceptance angle; and circuitry able to detect the rapidly-rising light intensity of any other flash being fired nearby. This closes its own sync contact, and triggers any flash attached to it. This approach requires at least one “master” flash to be triggered by a wired connection to the camera. It is also unsuitable e.g. at public events, where flashes from other photographers would cause spurious triggering.


Slave flash triggers arose as standalone accessories by the 1960s, or as all-in-one multipurpose flashes including both an electronic flash and an optical slave circuit in a single case.


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