Canon"s new Speedlite 600EX-RT is engineered to provide a new level of performance and reliability for professional flash photography with today"s most advanced DSLRs. With wireless two-way radio communication, the Speedlite 600 EX-RT allows photographers to expand their creativity using wireless speedlites with the reliability of radio triggering. The Speedlite 600EX-RT offers an expanded zoom range and a higher guide number, plus a number of improvements over its predecessor in both operation and feel. It features a new dot-matrix LCD panel for display of all pertinent shooting information, backlit controls plus an internal sound generator. Reliability is ensured with improved hot shoe contacts, improved flash head durability and the most comprehensive dust and water sealing system ever on a Canon Speedlite flash. A new color filter holder helps maintain accurate color in a variety of ambient light conditions. New wireless multiple flash system using radio wave communication for enhanced control of up to five groups of flashes. The Speedlite 600EX-RT represents the next generation in wireless flash systems. In addition to traditional optical wireless transmission, the Speedlite 600EX-RT facilitates radio controlled, two-way wireless transmission up to 98.4 feet, among up to 5 groups with a total of 15 individual Speedlites. Communicating on 2.4 GHz frequencies, the Speedlite 600EX-RT does not have the same directional limitations of traditional wireless optical transmitters. Where other wireless signals can be interrupted by physical obstacles, radio controlled systems excel. 15 transmission channels are available, selectable manually or automatically, and radio transmission IDs can be set to prevent misfiring in the event of signal interference on the same channel.