Hahnel 1000 799.5 - Viper Wireless Flash Additional Receiver Only for Canon (1000799.5). The Hahnel Viper is a sophisticated group flash trigger system that allows triggering of multiple flashes in multiple groups (up to 3 groups A, B, and C). Power output to each group can easily be collectively set by the Viper transmitter (sold separately), or alternatively each flash can be set to manually override such settings. Uses the 2.4GHz frequency and has Digital Channel Matching for the best possible reception and removing interference from other wireless devices within range