I got the Metz 44 AF-1 flash recently that is TTL capable for my Canon 550D, and have been reading around to get a better understanding of using flash on-camera (Neil Van Niekerk's article on flash exposure compensation was particularly helpful in this regard). TTL seems to be a useful feature and seems to take care of flash power based on the exposure settings. From what I've read, TTL seems to provide fill flash when in the semi-automatic exposure modes (Av, Tv etc).
I was wondering how TTL works in the manual exposure mode, where we can choose to deviate from the camera meter reading for correct exposure - what exactly would TTL be doing in such a case?
The reason why I'm asking the question is that it seems flash photography involves some amount of trial & error if we want to control the lighting, and understanding the way TTL works in Manual Exposure mode would make the approach more methodical.
Note: It seems that Nikon & Canon differ in the way their flash systems & Manual exposure modes work - Canon allows only flash exposure compensation when in Manual exposure, while Nikon allows both Flash & regular exposure compensation.
Update: Found another bit of information that may be useful for metering purposes when going with E-TTL\E-TTL II on Canon -
There's never any "full-frame" flash metering in E-TTL or E-TTL II. In both cases, with EOS cameras that use a 45-point focusing system/21-zone metering sensor, all flash metering is carried out by the 17 metering segments within the Area AF ellipse shown in the viewfinder. Subject matter outside the ellipse is completely ignored in terms of flash exposure control.