Chapter 2: Secrets of Canon Flash System learned the hard way

The effects of adding a second light source to your scene

Per given distance, move flash farther away from your subject to make the background brighter.

Place your autofocus point on the brightest part of the face for correct ambient exposure

In the evaluative metering mode, the camera uses many points to meter ambient exposure and give good exposure to the image. But does it do the same when adding flash? At the time of this writing, I have two Canon EOS 5D Mark III cameras. They each have 61 autofocus points that I can set and change focus points in several single and multiple pattern configurations. E-TTL II meters the zone (plane of exposure) not the focus point. Take my word for it. I’ve run several tests to prove this to myself. But the ambient light is mainly metered at the focus point. So it matters to have the focus point on the face when adding flash in Av mode.

You will still want to put the autofocus point on the face for correct exposure. As the ambient exposure (not the flash) is linked to it for correct metering.

So then what area of the scene does the flash pick to meter when not in program mode?

When you change out of program mode the flash defaults to metering at the center of your screen. On my camera, this is a small circle in the center of the viewfinder. The flash uses that circle to meter flash exposure. It can also change to this center circle metering of the flash in program mode if you switch out of evaluative metering. And the other way it can happen while you are still in program mode is to hit the FEL button (Flash Exposure Lock). It will then default to metering the scene at the center of the viewfinder as well.