The exposure mode determine the relationship between the flash and the camera. There are 4 different exposure modes when dealing with EOS Flashes:

  1. Aperture priority: Av
  2. Manual: M
  3. Program: P
  4. Shutter Speed priority: Tv
Mode Aperture Behaviour Shutter Behaviour
Aperture Priority: Av Set by the photographer Set by the camera for the ambient light: between a maximum X-Sync speed, 1/250 on a Canon 7D, and 30 seconds
Manual: M Set by the photographer Set by the photographer
Program: P Set by the camera for ambient light: between fully opened to full closed on lens. If it can’t balance the flash with the ambient light, then the camera will blink in the viewfinder and the ambient/background will be dark. Set by the camera for the ambient light: between maximum X-Sync speed, 1/250 on a Canon 7D, and 1/60s second.
Shutter Speed Priority: Tv Set by the camera for the ambient light: between fully opened to full closed on lens. If the camera can’t balance the light with the ambient light, then the camera will blink in the viewfinder and the ambient/background will be dark. Set by the photographer