Speedlite 600EX-RT: AF Assist beam

When using a multiple Speedlite setup, it is useful to know when your remote Speedlites have fully recycled and are ready to fire again. The ST-E3-RT will show a symbol on the display, however, for a quicker method; simply glance at the AF assist beam of the slave Speedlite and if it is blinking, the Slave is fully recycled and ready to fire again. If it is off, the Slave is still recycling and you should wait before shooting again.
EX Speedlites