11.7. Off-Camera Method 4–Wireless optical, sync-only: optical slaves

The simplest method to sync a flash unit wirelessly, if automatic metering is not required, is to use an "optical slave" light sensor. These sensors are quiescent when incoming light is at ordinary ambient levels, but respond to the bright pulse of light from a flash unit. They then trigger the attached flash unit.

Because optical slaves react so rapidly, the triggering is effectively instantaneous. A group of flash units triggered optically can be thought of as firing in unison, though of course there's actually a miniscule fraction of a second delay between the triggering (master) flash unit firing and the responding (slave) flash units firing. Any number of flash units equipped with optical slave sensors can be set about a scene for complex lighting scenarios. There are no limits other than budget and physical space. Figure 11-17