EOS5DC wrote in post #16675122
I agree with the conclusion of your first paragraph, if not the logic. It is not a 'signalling error'. It is the law of inverse squares at work. At longer distances you need either more light, or more sensitivity (ISO). A particular flash can only put out so much light. Your only other option is ISO. Use it. If it were a 'signalling error' my method outlined above would not work.
I can use my 100-400 at 400mm with a flash at 100 feet with high enough ISO, which pretty much kills your long-lens signalling error theory.
BTW, putting a flash in manual mode does not give it more power. Cranking up the ISO will give it more effective
Changing the ISO is a form of exposure compensation, and that can work. The reason it's needed is indeed inverse square-law, because the flash head cannot put out sufficient light. There is a signal error because with fully automatic ETTL, the pre-flash measurement cannot work when the lens focal length is longer than the flash head zoom -- they misalign. You can compensate with ISO, which as you suggested, effectively increases the usable EV, allowing exposure with the flash zoom at a hard limit. This is a form of gaming the ETTL system, and sure it can work.
The purpose of manual flash is to circumvent pre-quenching, given flash-head zoom vs. lens focal length mismatch. In other words, to defeat the ETTL system. This can work too, plus it can work more flexibly. You can manually choose the flash zoom angle to get the best far-end fill after (presumably) ceiling bounce, thus shaping the light. Then, you can manually choose the power level, to gain the EV needed. There are limits of course, and more ISO could still be needed, depending on the flash-to-subject distances. But taking ETTL offline enables the photographer to be in charge, to extend beyond the limits of automation.