wibbly wrote in post #11786963
Interesting that my question doesn't have a clear answer... the way this works just isn't intuitive I fear...
Ah, but it does have a clear answer. There's nothing random about the amount of FEC that's used for the exposure of each flash.
wibbly wrote in post #11786963
As far as I can tell the ST-E2 ratio setting sets a ratio (balance) between the flashes, so with no other changes the *total* exposure from *both* flashes remains the same. I don't know how it does this. Does it meter each flash separately (it ought to!), or just crudely set output levels?
The 580EXII allows me so set compensation even in slave mode (up to plus or minus 3 stops) - so I'm guessing this would be locked out in that mode if it was to have no effect? (The 420EX has no manual controls)
Each slave group fires a preflash that's used to determine exposure. Each slave group fire in turn, first A, then B and then C. Group C is not used to determine overall exposure, though, just A and B are.
Once the overall exposure has been determined from A and B taking into account the lighting ratio set on the master, the exposure for each of the slave groups is determined based on the ratio. Any FEC set on the master (or the camera if not set on the master) is then applied to each group exposures. The command is given to each slave group to fire at the predetermined level. Each flash then takes the exposure value given to it for its slave group and factors in the FEC that has been set on the flash and that becomes the resulting exposure from the flash.
wibbly wrote in post #11786963
I guess the only way to prove this is to take lots of test shots with wild FEC and ratio settings to see which have an effect, and which don't... I was just hoping someone had an authoritative answer or had done some tests already
There's nothing better than learning from one's own experiences. Learning by doing is a really good thing. 