Going back to Skip's original comments about the pre-flash and the Stofen reducing the amount of output, I found this comment about the LumiQuest Promax and now I'm totally confused:
"No exposure compensation is required when shooting with TTL exposure control, but SoftBox III will interfere with the on flash auto exposure sensor and autofocus assist. Approximate light loss is 1.25 stops."
If this device (and I assume the Stofen, too) reduces light, as in "light loss is 1.25 stops" how do they explain the "No exposure compensation is required when shooting with TTL"?
Does the modifier reduce the pre-flash by the same amount and therefore, no compensation for the final output?
Sorry, I guess I really don't understand, but unless the pre-flash is reduced, I don't see how the ETTL calculation "knows" there is a diffuser attached.
Any modifier used on a Speedlite will affect the pre-flash intensity in just the same way that it will affect the main flash burst. With most flash modifiers, that means that both the pre-flash and the main flash will be reduced by the same percentage due to the modifier.
Based on the Promax statement, I would think you would need to bump the FEC up by 1.25 stops - unless, as Skip said, it was already at 100% and there just isn't anymore power available.
What am I missing?
What you're missing is the fact that ETTL automation automatically compensates for the light loss due to a modifier (including bouncing light off a ceiling or a reflector panel).
Exposure Compensation (EC) and Flash Exposure Compensation (FEC) are only needed when the camera's "brain" goofs on exposures because of things like the color or reflectivity of the subject. The compensation is simply forcing the camera's exposure calculations to increase or decrease the exposure calculations based on the photographer's analysis of the scene and lighting.
Finally - if there simply isn't enough light to get the exposure up (or down) to a proper level, one can usually adjust the ISO setting on a digital camera to effectively increase the camera's sensitivity to light.