This Radio Control unit is suitable for the Speedlite 600EX-RT & later models. With its 30m range & sophisticated control panel, it’s an invaluable tool for complex off-camera flash setups of up to 75 speedlites. 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 & is powered by two AA batteries. Main Features 2-way wireless transmission 30m Range over 360° sweep Triggers up to 5 groups of up to 15 flashguns Sophisticated controls based on 600EX-ST flash including complex metering Supports E-TTL II, manual, strobe & automatic external flash metering LCD Panel with backlit Dot Matrix display 8 custom functions including 3 personal functions Dimensions (W x H x D): 67.4 x 61.5 x 77.4mm Weight: 110g (excluding battery)