Most of the Canon flashes have a big, monster red eye. It’s the auto focus assist light. On my 508EX-II, it’s made of 3 LEDs. When the ambient level of the light is low, it will illuminate to help the auto focus of the camera. But sometimes it doesn’t work.

The modern E series of the Canon flashes use a bar code pattern to illuminate the subject. These red and black stripes give enough of a contrast to allow the camera to focus. The maximum distance of the assist light depends on the flash, but it’s between 10 and 30 feet or 3 to 10 meters for the center focus point.

My experience is that as the focus point goes further away from the center point, the range of the auto focus assist light decreases.

As I mentioned earlier, sometimes the auto focus assist light doesn’t always work. Why?

  1. Light levels: if the camera can focus without the help of the assist light, it will not trigger the auto focus assist light on the flash.
  2. The focus mode: the auto focus assist light only works in the one shot mode.
    1. It doesn’t work in the AI Servo mode.
    2. It doesn’t work in the Live View mode.
  3. The focus point selected in the camera can’t be covered by the assist light.