photomad_2007: I've got a Sigma EF500 DG Super. Bought new in January it didn't have the firmware/chip upgrade needed to synch up in wireless mode. Prior to the fix, it would fire during the P-TTL prefire
before exposure, so wouldn't contribute to the exposure at all. So it was
consistent in offering no help on the flash. Is it possible your camera flash was giving the inconsistent lighting/underexposure, and the Sigma was simply not synching at all? In that case, you probably need the same upgrade I did, and many other users on this forum have reported.
Otherwise, I dunno. I've had great results since getting my flash back from the upgrade. But I agree that the guide book is TERRIBLE. Possibly you have a setting issue?
My Sigma, set to slave mode, looks like this:
Note the "SL" for slave mode, and the TTL upper left for auto exposure. And obviously the master flash (for me, always just the camera's built-in flash) set to wireless mode.
For what it's worth, I have my camera set to use the built-in flash to trigger the slave but NOT to expose the photo (option in custom settings). It might be harder to figure what flash is over/under exposing if both are contributing to the exposure.
And of course, different auto exposure settings will control aperture and ISO in different ways, which could exceed the Sigma's highest or lowest power output capacities, resulting in over/under exposure...
Hope that helps!
kyrios: yeah, the "magic" of TTL is pretty cool! I gather the trick to the "psychic" adjustments for distance and facing is that in the pre-exposure prefire, exposure information is exchanged BOTH ways, by not only the master pre-firing to synch the slave in time, but the slave pre-firing to demonstrate how much illumination it's casting on the subject, so that the master can then use camera exposure info to judge how strong a flash is needed and communicate this back to the slave, coded into a further pre-fire flash sensed by the slave. Pretty sneaky.
Oh, PHOTOMAD_2007: That reminds me, since wireless flash depends on the light of master and slave exchanged through prefire flashes, the ability of the slave and master to "see" each other's light is important. Too much extraneous light, too much distance between master and slave, and too obscured fields of view may lead to failures in wireless mode. Though again, I'd think that would more likely lead to a failure of the slave to fire at all (or to synch to the time of exposure) rather than to expose the wrong amount. But it's another thing that might be going wrong for you.