The ST-E3-RT Speedlite transmitter*1 uses radio transmissions to communicate with other Speedlites. It can control 5 groups and up to 15 Speedlite 600EX-RT slave units. The ST-E3-RT also makes use of a new dot-matrix LCD that provides detailed information on settings and features with a more intuitive user-interface and thus easier to view.
* Accessories subject to availability
*1 Because it does not have an optical transmission function, the Speedlite Transmitter ST-E3-RT is not compatible with earlier Speedlite models such as 580EX II.