The ST-E2 is a wireless transmitter for controlling Canon's own 420EX and 550EX flash units. It is light-weight and compact, and the battery can last for months or even years. It can also be used as a focus assistance light to improve auto focus in low light situations.
 

Wireless transmitter which can control up to Two Speedlite 580EX, 480EX groups set as slaves.

The transmitting range is about 12-15 metres indoors and 8-10 metres outdoors.

The ST-E2 fits onto the hot-shoe of the camera, and with cameras equipped with E-TTL II & E-TTL automatic flash exposure control, the ST-E2 can execute the following operations through wireless control :-

E-TTL II / E-TTL autoflash

High-speed sync

FE lock

Flash exposure compensation

Manual Flash (Flash output set with Speedlite)

Multi-flash firing (Set with the Speedlites)

The unit takes one lithium 2CR5 battery (not supplied)