11.8. Off-Camera Method 5–Wireless optical with automatic metering: Canon wireless E-TTL

E-TTL is the automatic flash control system used by nearly all EOS cameras since 1995. E-TTL's primary innovation was improved metering over TTL, but wireless capabilities were a key feature as well.

Wireless E-TTL is pretty well identical to regular E-TTL from the point of view of the user, the main difference being that the control signals are sent through the air from camera to flash unit by rapid pulses of light. No wires are required, just a visible line of sight between the devices (or reflective surfaces).