Vixen, here's something you can try. Set your flash in manual mode - than use the flash power setting to determine the "speed".
I understand you are doing macro photography and yes when you are really close up in macro mode any slight vibration will cause blurr and that's why you want high shutter speed. When speedlite is set to manual mode, the power setting determines how long the light pulse last - from 1/1000 sec at 1/1 power to 1/10,000 sec at 1/16 power to 1/40,000 sec at 1/128 power in the case of the Nikon SB700 (other brands differs.) Now, use that fact to capture the image at high speed.
1. Set your aperture for the depth of field you need for the subject and in macro, you don't have much leeway.
2. Set your ISO at the lowest setting (100 in the D7100)
3. Set shutter speed at anything below the FP limit - 1/200 or 1/250 is good. When using Flash in this mode, your shutter speed controls only the exposure of the background and not the subject of interest.
4. Remove all room light or background light.
5. Turn OFF speedlite and take a shot. You should see a black or almost black frame (you'll need to turn off AF also and manual focus.)
6. Now, turn on speedlite, set to manual mode - start at 1/8 power and then go up or down in power in subsequent shots to get the right exposure. If you go all the way up to 1/1 power and still is underexposed, up the ISO by 1 stop. and repeat.
The time length of the light pulse given off by the speedlite determines the exposure time of the image - not your shutter speed and given that even at the max power (longest time) the pulse is only 1/1000 sec, you are effectively capturing the image at 1/1000 sec to 1/40,000 sec speed - guarantee no blurr!! This is how high speed photography is done - we leave the shutter open (B-setting) in a dark room and some event (sound of gun etc) fires the strobe and capture flying bullets at 1/250,000 sec!!!
Now, this method works only if your background is far away - further than the coverage range of your speedlite or if not possible, have a plain grey or black seamless backdrop - sheet of felt is good.
Have fun experimenting.
