The ST-E2 can now be used as your on-camera master unit instead of a 550EX or MR-14EX. This allows an unlimited number of Speedlite 420EX and/or 550EX slave units to achieve precise main, fill and background lighting. The ST-E2 operates at up to 33' outdoors and up to 49?' indoors without cumbersome wires. It also permits E-TTL flash with ratio control of two groups of slave units controllable over a six-stop range.