For the first time, Canon has finally introduced a camera that can operate other eTTL flashes wirelessly without having a separate master flash mounted on the camera. The small built-in flash can be used as a “controller”. The pop-up flash will control either the 4xx, the 5xx series or many other compatible flashes like the Metz flashes.

All the flashes must be within a clear line of sight. The pop-up flash will do a “pre-flash”, a visible flash with the proper commands to the other flashes. The pre-flash happens while the mirror is still down and the shutter is still closed. Many people are confused and think that the pop-up flash is used to illuminate the subject, but it’s not.

The factors that influence the flash exposure:

  1. The distance between the flash and the subject of the scene.
  2. The reflectivity of the subject and the scene
  3. The tonal value of the subject and the scene

Inverse Square Law

The intensity of the light from the flash falls off with the square of the distance between the flash-and the scene/subject:

  • Twice the distance will give a light 1/4 as bright. Half the distance is 4x brighter (2 stops).
  • Three times the distance will give a light 1/9 as bright. A third of the distance is 9x brighter (almost 3 stops).