I have 4 600EX RTs and the ST-E3. The Canon ETTL-II TTL system seems to throw these off. When I have my camera in manual mode and the flash units on ratio the exposure values vary greatly as I move around on the set. I have not been able to figure this out. Over exposure up close and under exposure at a distance. That is what I had with my ST-E2 and my 580EX IIs. In order to overcome this I just change lenses to get the "sweet spot" for the flash. It is irritating enough to me to consider dumping them and moving to Profoto's new B1 strobes. Ideas?
Use them in Manual, ettl I still find completely useless..
ETTL is far from useless, but you have to understand what it is doing, what its strengths are, and how easy it is to completely confuse it.
The entire point of ETTL is to automatically deal with changing light to subject distances, not camera to subject distances. If you are using multiple static lights then ETTL will never give consistent results if
you move around a subject, the light on the subject changes and the flashes are trying to give a "correct" illumination of the subject.
Take this as an example. One light on a stand next to the photographer pointing at the subject, the subject gets even illumination and is an easy ETTL task, move the light back three feet and shoot again, ETTL deals with that effortlessly, put the light in a softbox, again ETTL deals with that effortlessly. Now, leave the light where it is but walk around the subject so the light is falling at 90º to them, sidelight. The ETTL exposure calculation is now basing the flash exposure on only half the light it was before, you will tend to get a one stop over exposure of the subject. Now walk around the subject a bit further, to say 140º, rimlight. Now the ETTL calculation is basing the flash exposure on only 10% or so subject illumination, it is going to blow out the edges by giving the flash full power to try to light the entire subject. This is not what ETTL is designed for.
If a light in ETTL is only illuminating a small section of the subject ETTL will "fail".
Now picture this, on camera flash for fill and two lights on stands either side of a dance floor where the subjects and photographer are moving. The subject distance to the remote flashes doesn't change much so they can go in manual, if the floor is small and the lights closer the distances change more so ETTL will work better. The on camera flash distance does change, put that in ETTL and you will get consistent flash exposures. Or put a light on the end of a pole with an assistant that kinda tracks the subject, ETTL will adjust for that too. As it will for bounce flash etc etc.