You can easily set up an E-TTL autoflash system with the 550EX, ST-E2 or MR-14EX as the sender unit and the 420EX as the receiver unit. Just set the 420EX's wireless selector to <receiver> and position the receiver unit(s) as desired within the wireless transmission range

• The 420EX and the 550EX can be set and used together as wireless receiver units
• If the 420EX receiver unit is not used for about 10 minutes, the SE mode will take effect (indicated by a blinking channel No.) automatically to turn off the power. Pressing the sender unit's test firing button within 30 minutes after the SE mode starts will turn the receiver unit back on.