« on: August 06, 2014, 11:48:02 AM »
Ok, I might be over simplifying this in my head, so please correct me if my line of thinking is off here....
Since the flash head zoom correlates to the focal length of the lens, at least in TTL, to find the appropriate flash zoom to fill a modifier, place your lens where the flash head would be in the modifier, if possible i.e. umbrella or mounted on the back of a softbox, and use the FL needed to just see the outer edges of the modifier. Then, the ISO and flash power settings would be adjusted for achieving the desired exposure. That way, the flash zoom for a particular modifier would be a consistent value. Again, let me know if this line of thinking is off the mark.
I don't know what zoom has to do with exposure. But yes, that is how I determined how wide I could go on my umbrellas. If it spills outside the umbrella, then it's too wide; go as wide as you can without spill. I never bothered to look at the highlight pattern within the umbrella, I just figured wider is better. With a brolly or a softbox it doesn't matter, there is no spill; I just set it to 24mm. For reference, I set my flash to 28mm with umbrellas. A couple of mine could go to 24 and a couple leaked at that setting so I just set them all to 28 so I don't have to worry about which is which.