Complete with built-in radio transmission wireless functionality, the Speedlite 600EX-RT sits at the top of Canon's on-camera E-TTL / E-TTL II compatible flash lineup with a powerful guide number of 197' at ISO 100 and 200mm. This two-way 2.4 GHz radio system provides added reliability and a range up to 98.4' among up to 5 groups with a total of 15 individual Speedlites. This system also does not need direct line-of-site like traditional optical-based transmission and will work through obstacles.