I'm trying to understand flash exposure compensation better. I get the relative scale, i.e., -1EV is half as much light as 0EV, but what is 0EV itself? Specifically:
At 0EV, is the flash trying to output enough light to expose the picture properly for the given settings with no ambient light?
What is the E-TTL system metering off of? Suppose, for example, that I have a scene that's correctly exposed with ambient light except for a portion that's completely shadowed. Then if I aim a flash only at the shadowed region and use 0EV, that region should be exposed similarly to the ambient region, right? But if that's the case, how does the system determine which part of the scene to use to calibrate flash output?