Wireless slave flash is a relatively new and wonderfully effective and convenient system for controlling a number of remote "slave" flash units. Remote flashguns can be precisely controlled in the manner required without the need to connect them all together with cables. Such a system is consequently a must for photographers wishing to establish creative lighting on location - or even in a studio.
Wireless slave flash is controlled by a so-called commander unit which may be be purchased as a separate dedicated unit or incorporated in a built-in or hot-shoe dedicated flash gun. Clearly all the units must be compatible with each other and with the relevant camera. The commander unit, whether built in to a flash gun or attached to a hot-shoe, sends signals to the remote flashguns to instruct them not only when to fire but also what level of output is required. Each slave unit must be set to "remote" mode, and to a specific group and communication channel, but can then be individually controlled from the commander unit. Each unit can be set to a different mode and output.
This degree of flexibility offers a photographer much more freedom in the way in which subjects are lit. Portraits can be improved significantly by placing a slave flashgun out of shot and directed towards a potentially dark are of an image. A slave unit might be used to illuminate a background and hence remove unwanted shadows, or directed towards the back of the subject's head and shoulders to produce attractive rim lighting and golden back-lit hair. Given that all this can be done automatically with only the simplest of settings, it is difficult to imagine how we managed with anything less.
Remote slave units can be arranged in groups and controlled using individual communication channels allocated by the commander unit. A typical set-up might have one flash unit in a first group and two in a second group. Flash output levels can be controlled or changed from the camera's commander unit without having to walk to the slave unit. More sophisticated systems allow a photographer to adjust lighting ratios in an image remotely. There is no need to maintain a line of sight between the commander unit and the remote flash units, and the slaves emit a single beep to tell the photographer that they have fired.