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I know one can use a ND filter, to be able to get the background right even with big apertures (during daylight and shooting at synch speed) and using the flash/strobe to light the subject in the desired way.

The same can be achived by using HSS and and shutter speeds shorter than the synch speed.

Just to get the full understanding: Only to freeze fast moving subjects which need shorter shutter speeds than the flash's burning speed HSS is really required?

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Just to get the full understanding: Only to freeze fast moving subjects which need shorter shutter speeds than the flash's burning speed HSS is really required?

Sort of. That is a common reason to use HSS, but there are also other use cases where HSS may be preferable to using ND filters.

  • When you also want to use narrower apertures interspersed with wide apertures during a shoot and don't have the time to add/subtract the ND filter(s) between different aperture settings.
  • When the subject is static but other things in the background are not. Think about things such as leaves on trees or wind chimes on a porch that are in the frame with your portrait subject during a windy day. The short exposure time (Tv for "time value") will freeze things in the background that may be distracting if they are blurred even if your static subject is not.

It also depends on exactly how short of an exposure you need. HSS loses more power for a shutter time of, say, 1/8000 than it does for 1/1000. It may be more efficient to use ND filters and get brighter flash output to freeze the subject with a slower Tv than to use lower powered flash output at very short Tv without an ND filter. Where the "crossover' point is depends on how bright the ambient light is compared to the total flash power available.

  • For clarification, HSS doesn't loose more power at higher SS's; you're just recording less of it... the flash is acting as a constant light source. – Steven Kersting Nov 4 '19 at 22:42
  • The effect on the photo is less total energy from the flash captured by the camera's sensor, even though the duration of the time it takes the shutter curtains to transit the sensor is the same for anything over sync speed all the way to minimum Tv. It's usually about 2.5-3.5 milliseconds, depending on the specific camera. – Michael C Nov 5 '19 at 12:54
  • Yes, the opening between the shutter curtains gets smaller; the 2nd curtain starts following sooner. The power loss/difference occurs at the beginning when the flash first switches into HSS mode. You just won't notice it unless you need the extra power. – Steven Kersting Nov 5 '19 at 19:02
  • The power loss occurs because more of it is bouncing off the shutter curtains and less of it is hitting the sensor. – Michael C Nov 30 '19 at 10:16
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The problem with "freezing fast moving subjects" using shutter speeds faster than the flash sync speed is that the flash sync speed corresponds to a fully open shutter, where the front curtain has left the picture and the back curtain has not yet started moving. "Faster" shutter speeds are actually the front curtain travelling ahead of the rear curtain by the shutter time. Which means that the "freeze" of a fast moving object is actually a "relativistic" distortion of it. Using an ND filter instead to some degree replaces the distortion with motion blur. If an object is not frame-filling, the effect of the movement distortion will be moderate at least perpendicular to the shutter motion. In that case, HSS might provide reasonable payback.

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In order for HSS to provide the multiple flash pulses from a single capacitor charge the flash's max output is automatically reduced to about 1/4 power. That might not be enough to drop a bright BG as far as you would like. From that point on the shutter speed cuts the flash output and ambient equally.

But because the ND cuts the flash output and ambient light equally right from the start, and because the flash is able to operate at full power, there's the potential for a greater ability to drop the ambient with the desired settings... i.e. the flash at full power is a closer match to the ambient/BG lighting.

Edit: IDT I actually addressed the question... how they compare for freezing motion. HSS simply converts the flash into a constant light source like any other room/ambient light, and you use shutter speed to control the exposure and freeze motion just as you would w/o flash.

In order to freeze motion with flash the exposure settings must result in very nearly a black frame w/o the flash firing. You then need the flash to fire with a T.1 time fast enough to freeze the motion. The ND helps in creating the dark exposure.

With a traditional studio flash/strobe the ND additionally helps by allowing the strobe to fire nearer max output where it's T.1 time is faster, but typically not fast enough to freeze high speed motion.

With modern IGBT controlled flashes/strobes the T.1 time decreases as the power is reduced, and they can reach speeds much faster than other types. However, in this case the ND hurts because it also requires the flash to operate at a higher power and slower T.1 times.

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