Originally posted by maxfield_photo I think HSS on my Metz 58 works at quarter power down to 1/256th power. So that would be two stops under full, maybe other units are different, but if I crank my shutter up to 1/4000th, that's a lot of ambient-light-killing power, yet the light from the flash is unaffected. I don't know, you may be right, I just got the Metz 58 a month ago, I haven't had a chance to really compare the two methods.
No, in HSS mode, the flash behaves just like a continuous light source, so any increase in shutter speed will just eat power from it...
After all, once the flash hits its maximum power in HSS mode, doubling the shutter speed will halve the amount of light provided by the HSS flash (the traveling slit is half the size it was!).
So at 1/4000th, your Metz 58 will mainly illuminate the shutter itself, and the amount of light actually hitting the sensor will be very small.
Some flash manufacturers provide the maximum GN delivered at given shutter speeds, and doing the math, you can conclude that using a ND filter eats less power than using HSS... See the last page of
the Sigma 610 Super manual.
Quite easy to understand, as they both behave the same way once you passed the first step (adding a one-stop ND filter will give you exactly the same results on ambient and flash as doubling the shutter speed in HSS), but in HSS mode, the flash will lose some power by firing slightly before and after the exposure, and because of the electronic constraints caused by the sinusoidal wave modulation of the flash output.