Do not set it if you have an EOS Digital camera or the EOS Rebel T2/300 X . ... If "AF- assist beam OFF" is set with the Speedlite or camera, the AF- assist beam will not be emitted.
The Speedlite 220EX AF-Assist beam is emitted automatically to enable precise autofocus under dim light and low contrast conditions. The AF- Assist beam is effective from 1 to 5 meters (3.3 to 16.4 feet)
5 . Confirm that the <LINK> lights on the sender and receivers are lit and green and that the AF- assist beam emitter on the receiver is blinking at 1-second intervals. Also confirm that the red pilot lamp (ready-light) on the ST-E3-RT is illuminated.
Direct- sync, shoe-mount flash with E-TTL/TTL autoflash control and wireless transmission/reception (E-TTL preflash, AF- assist beam, auto zoom, and bounce flash). Compatible Cameras
In low-light or when the viewfinder image gets darker at high magnification, it becomes difficult to achieve correct focus. In such cases, the focusing lamp can assist with focusing.
Set the camera's shooting mode to manual exposure mode (M), then set the desired aperture value and a shutter speed between 30 sec. and the camera's X -sync speed. Make sure the TTL indicator is displayed in the flash unit's LCD panel
