Creating Portraits never got easier since Canon updated their flash system when setting it to auto program mode. This is called Evaluative Through the Lens Metering (E-TTL).
Excerpt from “Secrets of using Canon E-TTL On-Camera Flash for Better Portraits”
E-TTL II is a software improvement on E-TTL and is now a standard in all EOS cameras introduced with or after the Canon EOS-1D Mark II in 2004. E-TTL II is implemented in the body, not the flash unit, and therefore can use existing E-TTL flash units and EF lenses.
Why Canon switched to E-TTL II
Both systems used a pre-flash but when using the original E-TTL flash system the flash exposure was linked to the autofocus point. With the center autofocus point being the most accurate, most photographers would focus using the center point on the face, lock focus (holding the shutter button halfway down) and re-crop (move the subject in the viewfinder) then take the shot. A lot of the time the autofocus point would fall on the background causing overexposure. With E-TTL II Canon unlinked the flash exposure from the autofocus point. Instead they tied the flash to the camera’s evaluative metering zones. In addition, they added a distance measurement taken from the lens to help calculate the flash exposure. This was especially a big benefit to Wedding Photographers when they photographed a bride in a white wedding dress. The exposure was more accurate and therefore they could concentrate more on posing, etc.
Note: You still don’t want to do the center focus point and re-crop thing and shoot as the autofocus point is still tied to AMBIENT EXPOSURE!! So when in program mode move the autofocus point (not the camera) for more accurate flash/ambient exposure.
To Learn more about the Secrets of using Canon E-TTL On-Camera Flash for Better Portraits course, click on the link below.