The Guide Number Controller is an auxiliary device that mates with camera or housing and the dedicated YS-350 TTL/PRO strobe. Dual control dials allow subtle manipulation of single or dual strobe output in 1/3 EV increments from full to 1/16th power. The Guide Number Controller enables the highest level of manual flash exposure management for nuances in illumination, Compatible with camera systems equipped with 5-pin bulkheads.