Thought this might be of interest to some. Probably not many though. Warning. Very long!
But first I have to confess that it is actually an accidental bonus to what I was trying to achieve. My primary aim is to capture dragonflies in flight with the details of the wing veins clear. For this it is necessary to freeze the motion of the wings. Whilst this can occasionally be done with a relatively slow shutter speed, my experience shows that even a shutter speed of 1/2500 can be too slow. So therefore I try to shoot at 1/4000.
Then I also want all of the wings and the abdomen to be in focus, if possible. And I don’t want the details to be compromised by either diffraction (so I use f/8 for preference) or noise.
When I first started doing this (2008), I was using a Nikon D300, and shooting at 400 ISO. Problem. The sun wasn’t bright enough to use those settings. I experimented with ISO 1600 (hopeless) and ISO 800 (required spending far too much time in the digital darkroom selectively sharpening some areas and selective noise reduction on others). So I had to compromise with my settings, and rarely got an image that I was really pleased with (well, I would get plenty of nice pictures of dragon-don’t-flys, as I call them when they are perching, but it’s the flight shots that caused me to by the camera in the first place - that’s what excites me about photography). It got to the point that I rarely bothered to take my camera out at all. 
Then Nikon released the D400 (OK, they called it the D810, but it had a crop mode). Suddenly there was usable ISO 3200 and I could use my settings of choice. True, it could only go up to 7 fps, as opposed to the D300’s supposed 8 fps (a high frame rate helps tremendously, as the position of the wings cannot be predicted. So the more frames I get, the more chance of a pleasing picture). But everything else was a clear improvement (in fact my measurements of the frame rate I can achieve with the D810 is about 7.1 fps, and I never seemed to actually achieve 8 fps with the D300, so I’m not convinced that even that aspect was worse. Of course, if I was buying now I’d get a D500).
But that wasn’t the end of the story. I said ISO 3200 was usable, but it was far from ideal. Even shooting in crop mode, I’m cropping in post-processing routinely. The reason is simple. Dragonflies fly fast. There are two big difficulties in photographing a dragonfly in flight. The second one is focussing. But the first is to point the camera in the right direction (it is true that some dragonfly species will often hover for long enough to give you time to leisurely frame and focus, but I want to photograph all species, all of the time, in all their different flight modes - I’m a glutton for punishment, it seems). So the further away the dragonfly is, the easier it is to follow with the camera (smaller angular movement). But then if it is far away, it is tiny, and cropping to fill the frame shows the noise up horribly. So yes, the D810 made a big difference, but still I wanted more.
At that time I was either shooting with a Nikkor 105mm f/2.8 VR macro lens (for those rare occasions when I could get close enough) or a Nikkor 300mm f/2.8 VR (for longer distances). The latter is superb optically, but isn’t great for hand-holding for long periods, and has a high inertia (meaning I can’t swing it quickly to follow the dragonfly). And then last year Nikon released the Nikkor 300mm f/4 PF. Almost the same size and weight as the 105mm, this was a godsend to me. Now I can swing and follow at a distance as easily as I can for those closer shots with the macro (which I rarely use now for flight shots). My keeper rate shot up again.
Still, I wasn’t satisfied. It would still be nice to get the ISO down to 800 or even 400. So last winter my thoughts turned again to flash. I had always dismissed it as too slow (the Speedlite SB-910 has a full-power duration of 1/880 sec.). Wait-a-minute. Full power? I thought again. I might shoot at reduced power. But would it be bright enough, and would it keep up with my camera at 7 fps. Re-cycle time 2.3 seconds (hopeless) for full-power discharge. Oh. But if I’m firing at reduced power so as to get a flash duration of 1/4000 or shorter?
I downloaded the manual, and started to do some calculations. I was willing to compromise with a maximum range of 20 feet / 6 metres (as experience told me when I was shooting at 30 feet the resulting deep crop was never going to be first class anyway). Still the numbers wouldn’t come out.
Hm. The numbers look three times better when using the (expensive) SD-9 battery pack. Still not enough. What about that better-beamer thing that I’ve seen advertised. Two-to-three stops better range, they are saying. But that’s with the flash head set at the 50mm zoom position (for a 300mm lens). But I’m using DX crop, so my 300mm lens has the field of view of a 450mm lens on full frame. So maybe I can set the zoom head to the 200mm position and still use the better beamer. ISO 800 at 20 feet (or 400 at 10 feet) seems doable. If I can trust my extrapolations of the specifications.
So I started asking around. First on DPreview (no, it won’t work, said everyone). I tried contacting Quantum about their Trio flash - “you’ll need multiple flashguns” they said. Too impractical.
I went over my calculations for the SB-910 again. It all depended upon my assumptions about the re-cycle time - would it scale to my proposed settings? Nikon didn’t say. I asked Thom Hogan. Not predictable, he said.
Of course, there was one way to find out. Go ahead and buy the equipment and try it. But this was taking a chance with my money, and everyone was telling me it (probably) wouldn’t work.
So in the end I went ahead and did it. First thing I did when the equipment all arrived, was to take pictures of my wall (it was winter - no dragonflies to shoot) from 20 feet at 7 fps (crop mode, 300mm lens). It worked! Every frame perfectly exposed for a 15 frame burst.
15 frame burst? Nikon warn that the flash head can overheat if shot at that rate for longer. And let it cool for 10 minutes before trying again! And 3 days after I made the purchase then announced the SB-5000 which doesn’t have that problem 
So I have my action-flash settings limit burst mode to 15 frames. And so far it hasn’t been a problem. If it becomes one, I have an upgrade path (and this time I won’t be spending money blindly). meanwhile I can happily shoot at ISO 800, and darktable’s profiled noise reduction does a fine job. or even at ISO 400, as I did today, for closer shots (and that doesn’t even need the noise reduction).
