Originally posted by amoringello People keep making an argument with NDs when these questions come up.
Easy enough to test. Try it. You'll see pretty quickly that ND affect both flash and ambient equally and therefore has no positive effect on freezing action. If you had the ability to turn up the flash to overcome ambient, shutter speed would not be an issue... and therefore neither would use of NDs.
???
The purpose of ND filters for freezing action is to lessen the ambient to the point it doesn't participate to the pic anymore... Then the action is lit by the flash only, which is quite instantaneous (usually 1/1000 for a full power blast, but even then the main part is more like 1/2000)...
You can indeed do the same by closing the aperture, or lowering the ISO, but if these two are already at their limits, ND filters are the only real way... HSS does not freeze the action (as it still takes 1/180s to expose the whole sensor), and higher sync speeds (with leaf lenses or digital shutters) are useful only up to 1/1000, where the flash starts to behave like a constant light source instead of a burst.
Granted, ND filters only work up to the maximal power of the flash...
Talking about deserts, Joe McNally once did a
7 strobes shooting in the desert using HSS.
Doing some simple calculation, he could have done the same shoot with only 5 strobes by using a ND filter. HSS does eat a lot of power...
Now, if you have to use a wide aperture, and you're already a min iso, then a ND filter will do more for you than a half-stop improvement of the sync speed.
As the only reason to use a faster sync speed is to further lower the ambient when iso cannot, it means you're in a well-lit situation, so the dimmer viewfinder should not be troublesome in this case.