Optical Slave
A sensor that fires a flash when it detects the light from another flash.
Measured in
m (range)
In depth
Optical triggering is cheap, universal and requires no matched hardware, but it needs line of sight, struggles in bright sun, and can be set off by other photographers' flashes. A digital slave mode ignores the metering preflash of a TTL system, firing only on the main burst — without it the slave fires too early and contributes nothing.
Examples in practice
A background light on optical slave fires reliably from a key light's burst in a darkened studio, with no cables at all.