The Nikon SU-4 Wireless TTL Slave Flash Controller enables wireless remote TTL slave flash with any current Nikon Speedlight. The SU-4 detects the firing of the master flash (including built-in pop-up flash) on Nikon cameras, and sends a firing start signal to the slaved unit. The SU-4 stops the firing of the slaved flash by detecting the firing stop of the master flash. Use of SU-4's eliminates connecting cords. Operates up to approximately 23' from the location of the main flash.