The canon st e3 rt speedlite transmitter can fire up to 15 compatible speedlite flashguns in three groups over distances of up to an impressive 30m radio frequency control provides reliable operation even when direct line of sight is not possible the st e3 rt can be used in almost any environment thanks to its weather proof construction the canon st e3 rt radio only transmitter has no optical mode so cannot be used with older non rt flashguns such as the 580 exii and is powered by two aa batt