Very good trick, that bounce! Sure does not look like dim ambient light! I suppose that was your 70-200 f/2.8?
1) Back to AF assist: some elementary mechanics: i am a bit confused. In each case, camera is in AF-S. Suppose I want the flash only for AF assist. (After i sent the prior message, i realized that diffusion should be irrelevant for AF assist. The only issue is whether the focus circuitry gets enough light.
2) Re focus trap: another trick. I do use focus priority all the time, but I never though of holding the shutter release and letting the camera decide when to shoot, when it decides that the subject is in focus.
3) Related to that, can't we trust that "in focus" dot in the viewfinder? i was relying on that for confirmation of focus before I shot. (Small area focus setting.) You said that I might have thought I was in focus when I was not. Also, the camera was at all times in focus priority. Does manual focus override focus priority, so that the camera would shoot even if the focus was not accurate?
(1) The flash is on the hot shoe. Does commander mode apply? Your test suggests that it does not matter whether commander mode or half press is used. Is that correct?
(2) The flash is remote. same question.
(30 I saw my niece, the dancer, this evening. She immediately reacted negatively to using flash. Then I explained that she would not perceive it if it is only for AF assist. That would work. I told her of our discussions. when I told her that you could predict the positions of professionals w/in two inches, she was not at all surprised.
(3) As to bounce flash, that would be politically possible during rehearsals. The problem here is that the theater ceiling over the audience is two stories high (not so much by theater standards), and the ceiling over the stage is higher, and full of lights and drops. Who knows where the scattering would go. I think only the ceiling over the audience might help. If the flash were pointed up at 45 degrees, I might get a lot of bounce light onto the stage. Certainly worth experimenting!
4) Speaking of "a lot of light", when I read the specs on the SB 900, it didn't appear to have a "really significant" increase in power over the SB-600.
5) Another issue is that I can only get access during dress rehearsals, which is also the only time I can get action. (But it has occurred to me that there are other shows than ballet that have rehearsals there ...)
6) The theater rules seem to have been changed to allow the art director to permit photos from the audience during a live performance, but I would have no freedom of position. Previously this was forbidden. The usher claimed that the LCD was distracting to neighbors.She was not the type to show that I could dim it significantly. Somebody had made a rule.) But that would still be absolutely w/o flash in any case.
7) Re that 85 mm. Not only is it 1/5 the cost of the 70-200, it is 1 1 /3 stops faster. I just saw that the 85 weighs 330 g, the 70-200 weights 1540 g. Thus, x 5. Is Nikon pricing by weight? When I held a 70-200 mounted, in the camera shop, it did not feel as heavy as I had expected. Bjt that was only for a few minutes. At baseball games, I am hand carrying the camera with an SB-600, and after an hour, it gets heavy. I think I need to use my monopod at the games.)
I still feel that i need to check out the practically of the field of view of the 85.