With a 2.4 GHz radio system, a powerful guide number, and full manual operation, the Speedlite YN660 from Yongnuo is an excellent tool for photographers who desire the utmost control from their lighting equipment. This radio system builds upon the previous generation with a total of six groups and 16 channels, as well as a range of 328'. This system can also be used as a master trigger for YN660, YN560-IV, and YN560-III flashes as well as RF602, RF603 II, and RF605 triggers for maximum compatibility. In terms of pure output, this manual flash has a rated guide number of 217' at ISO 100 and 199mm and has a zoom head with a range of 20-199mm. This zoom range can be expanded to 14mm with use of the pull-out diffuser in the head.