When the picture is taken in which the ambiant light in the background is properly exposed and along with that a flash is used to illuminate the subject; we say its a fill-flash picture. When these two exposure, ambiant and flash are kept balanced to get a natural looking picture; such a technique is called Balanced Fill Flash.
In Canon's EOS System; one doesnt has to do anything special...as system is doing two metering; one for ambiance and the other for flash seperately. All you need to do is keep the mode in P (Program), Tv (Shutter Priority) or Av (Aperture Priority) and use either the built in flash or the Speedlite in ETTL mode. When flash is being used in these three modes; camera automatically keep the shutter speed to 1/60 or more to avoid blur and it even automatically reduce the intensity of the flash output when ambiance is properly exposed.
If you have a properly lit ambinace (outdoor / sun); you can dim the background if required by using exposure compensation and if subject is getting over-lit with flash you control it with Flash Exposure Compensation. When using Speedlite and clicking indoors; keep ISO 400 or 800 for more light-gathering.
- Evaluative Metering works best with flash mostly
