If you turn on the flash and you are in Program mode and if the average light reading is fairly bright, the e-TTL-II will default to fill-in flash. Canon’s fill-in flash setting automatically applies a -2 flash exposure compensation for the Canon 7D. No user input is required.

  • What’s a fairly bright light reading? Around 13EV or f/8 @ 1/125s @ ISO: 100 or brighter. It’s a sunny day or an bright cloudy day. Exposure Values are usually expressed at ISO: 100
  • If the ambient light is much lower than that, the shutter speed will be set between 1/250s, the x-sync speed for the Canon 7D, and 1/60s. The flash will expose for the foreground and let the background become a cavern. It starts around 10EV and lower or f/4 @ 1/60s @ ISO: 100

This leaves the question of what happens between 10EV and 13EV? It’s a blend and the Canon EOS flash will apply from 0 flash exposure compensation to -2 flash exposure compensation as it gets to close to 13EV.