Originally posted by Class A Your expectation is incorrect. I hope mcgregni's explanation already makes sense to you. Only if an automated exposure system actively compensates for higher shutter speeds (by amping up the flash output) then you won't notice a decrease in flash exposure. The natural effect is for the flash exposure to decrease by one stop with each stop of shutter speed increase. Not that fault of the flash or triggering; just that the fact that a lot more light just hits the shutter blades (as the open slit gets smaller and smaller).
Don't know how HSS or hypersync work, but shutter speed on normal flash does not affects flash exposure. Unless, like I said before, your shutter speed is faster than the flash duration, not my case with 1/400.
It doesn't matter that the slit gets smaller and smaller, it still gets whatever the shutter speed is of time or exposure(light), so, as long as the flash fully triggers in less than that time, it should fully expose the sensor or film just as much as in any slower shutter speed.
And like I said, is not only the exposure that changes, it's the flash output, as you can clearly hear the flash sound louder at sync speed or below. So, HSS in this setup must be limiting the flash output.
PS: after reading how HSS works, it does need to limit the flash power, since it emits extremely fast pulses of light, so it cannot be full power, or it wouldn't keep up with the speed.
When using HSS with my setup, the flash exposure doesn't change when increasing the shutter speed, at least not from 1/200 to 1/1000 the fastest I could test it today, it only changes when going to 1/125, because it allows the flash to trigger at full power, slower than 1/125 doesn't change the flash exposure from 1/125 as well,