The Canon Speedlite 600EX-RT Flash is a versatile, compact lighting unit that can be mounted onto the hot shoe of your Canon camera or fired wirelessly from either your camera's built-in flash optically or via radio with an ST-E3-RT transmitter (not included). This flash improves on Canon's previous flash, the 580EX II, by adding radio transmission to the once infrared/optical-only triggering system. Radio transmission allows for easy master-slave communication between other 600 EX-RTs or with the ST-E3-RT transmitter, even when the transmitters/receivers are not in line-of-sight of each other. This, along with increased range, control over a larger number of flash groups, and a wider and longer zoom, makes this flash the perfect choice for wedding photographers working alone or with assistants. It is also ideal for portrait and travel photographers who need to create the biggest light possible from a very small unit.