This is a long article, but here's a short version first:
HSS is typically a high-end feature, and it requires both the camera and (a compatible) flash to BOTH be designed to implement HSS mode. HSS flash (High Speed Sync flash mode, the Nikon menu calls it Auto FP mode) is an entirely different flash mode, for focal plane shutters. HSS flash is triggered repeatedly and very rapidly, so that it mimics being "on" like continuous light (like sunlight or a desk lamp is on continuously). As seen by the shutter, HSS is continuous light, like sunlight (meaning, the flash is on when shutter opens, and still on when shutter closes). Human eyes do still see a "flash" (the shutter travel duration), but this continuous "flash" stays on all during the total focal plane shutter travel time.
However make no mistake, HSS is NOT high speed flash. It is merely called High Speed Sync, since continuous light has no sync requirements. HSS no longer works same as "flash", and is no longer a speedlight - continuous light has no motion stopping capability at all (we do still have the shutter speed, but the speedlight is typically faster).
The continuous HSS flash mode is implemented by pulsing it tens of thousands of times per second, appearing continuous, from slightly before the shutter opens until slightly after it closes. For example, perhaps HSS is on for 1/200 second (5 ms shutter travel time) instead of a speedlight (at 1/16 power) for 1/10,000 second (0.1 ms). To be able to be on continuously so much longer (50x longer in these numbers), it can only run at about 20% power level maximum.
1/400 second at f/11
1/800 second at f/8
1/1600 second at f/5.6
1/3200 second at f/4
1/6400 second at f/2.8
1/200 second at f/16 normally would be in this list, but it was omitted because 1/200 second would not enable HSS flash mode (very different).
So of course, HSS has the properties of Continuous light (it is continuous light), instead of the properties of speedlight flash. HSS exposure is affected by shutter speed, same as sunlight is affected.
The usual reason to use HSS flash mode is to provide wide aperture (to blur background), by allowing fast shutter speeds for fill flash in bright sun, at perhaps 1/3200 second at f/2.8 (ISO 100) - if desired, and if the reduced power (20%) allows sufficient distance range (40%). Flash models vary in power capability, but if HSS distance is approaching ten feet, start being concerned about sufficient flash power.
So, while HSS can use any high shutter speed, a fast shutter speed duration will tremendously diminish the continuous light captured (same as sunlight does). However, since it is continuous, Equivalent Exposures apply again, which now applies to HSS flash too, same as sunlight, meaning a wide aperture can compensate for the light losses of a fast shutter speed, for both HSS flash or for ambient light, since both are continuous. To be able to use wide aperture with flash is the usual thought (allowing fill flash in sunlight at f/2.8). In contrast, the problem is that regular speedlight flash mode is limited to a slower shutter sync speed, which in bright sun, forces a greatly stopped down aperture, perhaps f/11.
Indoors, in dimmer light where we need flash, we can use wide aperture anyway, so HSS flash would be pointless and counterproductive and inappropriate in typical indoor scenes. The regular speedlight mode will run circles around it, power, range, speed. But in bright light, the focal plane shutter is not fully open to sync speedlights at shutter speeds higher than about 1/200 second (ballpark maximum for many cameras). This does not matter indoors in dim light, and the speedlight can be much faster than any shutter.
In bright sunlight ambient, flash mode TTL BL should provide the proper flash fill level automatically (point&shoot fill with camera P mode). Or TTL mode or Manual flash will need to be compensated about -2EV to be fill flash in bright sun. Since fill flash is reduced level, then as such, -2 EV fill doubles the range of brighter flash seeking 0 EV as main light. And very differently, in this HSS mode, any Equivalent Exposure combination will give the same exposure, for both sunlight and for fill flash. Regular speedlight flash certainly does Not work like that. :)
This HSS flash mode does NOT change the way the shutter sync works, instead it changes the way the flash works. The flash has to be built to implement this continuous HSS mode. The camera has to be built to enable and trigger and control it. The HSS selection is on the Canon flash unit, or in the Nikon camera menu. The camera and flash communicate about this, a matched system. This system communication (and thus HSS mode) is not possible except on the hot shoe or via Commander. Manual or TTL flash is possible with HSS then. The Auto FP feature is typically not offered on entry level flash or camera models. Both flash and camera have to provide it.
Note: The Nikon camera internal flash cannot do HSS flash, but it can still be the Commander used to trigger HSS mode in remote flashes. However, since it cannot do HSS itself, its own light contribution must be disabled (Built-in flash group Mode set to "- -" in the commander menu). We are not allowed to set shutter speeds faster than maximum sync speed until the internal flash is disabled (in Commander menu), or until the internal flash door is shut. Then, even when disabled, the internal flash Commander still flashes commands, before the shutter opens, so the flashing will not appear much different to humans, but if disabled, it will not contribute lighting into the picture after shutter opens.
This topic is more special purpose (Not basic flash, and some models do not offer HSS), so no harm is done by skipping this page for now. Next Page - Soft Light.
There is a Guide Number calculator just for HSS flash.
|Speedlight mode||HSS mode|
|Shutter speed cannot exceed the cameras Maximum Shutter Sync Speed, typically about 1/200 second.||Allows any fast shutter speed that the camera can do. There is no sync issue for continuous light.|
|Flash indoors is no issue, but maximum shutter sync speed of 1/200 second requires flash fill in bright sun to stop down to about f/11 (ISO 100).||Allows wide aperture with flash in sunlight. HSS in bright sun might use f/2.8 at 1/4000 second (ISO 100).|
|Flash duration is faster than shutter speed, so speedlight exposure is independent of shutter speed (up to maximum sync speed.)||HSS is continuous light, so Equivalent Exposures apply to HSS too. Faster shutter speed is necessarily compensated by wider aperture (like in sunlight).|
|Range of Guide Number depends on aperture: Distance = GN / fstop||Same GN and range for any Equivalent Exposure.|
|Very short (very fast) speedlight duration can freeze very rapid motion (called Speedlight). Lower power levels are faster than any shutter speed.||Continuous light cannot stop any motion. HSS can only use fast shutter speed, reducing the light, requiring wide aperture.|
|Speedlight can use 100% of flashes full power level.||A biggie - To be continuous, HSS only allows about 20% power (-2.3 EV), resulting in about 40% distance range.|
(First two can be advantages for HSS. Last two are big advantages for speedlight.)
Nikon flash manuals call it FP High Speed Sync, but their camera menu calls this feature Auto FP flash (Focal Plane mode). Canon calls it HSS (High Speed Sync) and sets it on the flash, but same thing. Here, I tend to use the names HSS and FP mode interchangeably. Auto FP does not actually shift into HSS mode until the actual shutter speed is faster than the Auto FP menu number. The HSS mode allows use of any fast shutter speed with "flash", by drastically changing the nature of the flash unit itself, to no longer be an instantaneous pulse, no longer fully powered, no longer a speedlight, but to instead be a continuous light (for duration of focal plane shutter travel). This HSS flash mode is a feature of higher end system gear (both camera and flash must provide it).
Cameras with a focal plane shutter have a maximum shutter sync speed with flash, limiting the fastest possible shutter speed we can use with flash. Today, the maximum shutter sync speed with flash is typically around the 1/200 second ballpark (can vary a little with camera model, but it affects all models with a focal plane shutter). This is not much issue for flash indoors (the speedlight is much faster than the shutter), but it becomes an issue with fill flash in bright sun (explained here). To aid that purpose, top DSLR models and speedlights do offer an Auto FP - or High Speed Sync mode (HSS), which does allow any faster shutter speed with flash - any speed faster than the maximum sync speed is possible - a radically different mode, but with reduced distance range. The flash does this by becoming a continuous light, meaning it is on when the shutter opens, and is still on when the shutter closes. Humans will still see a quick flash (for the shutter duration), but as seen by the shutter, it has been on continuously (like the sun or a light bulb always appears on). This is a very special flash feature, and the Nikon menu feature of Auto FP (often menu E1, Flash Sync Speed) is the camera option that enables it.
Not very often, IMO. There seems only two conceivable purposes of HSS flash, both involving bright ambient (sunlight).
But this reason makes no sense at indoor ambient levels, because f/2.8 is no issue at all for speedlight mode at indoor light levels (just turn the speedlight power down sufficiently, which makes it even faster).
In any not-excessively-bright ambient (like indoors), the regular speedlight mode runs circles around HSS mode (speed, power, range). At 1/4 power level, the speedlights (Nikon SB-700, SB-800, SB-900) are spec'd from 1/2550 to 1/3500 second duration (and HSS mode runs at only about 20% power). At 1/16 power, specs are 1/10000 to 1/11100 second. And much faster at even lower power levels. Speedlights are how high speed photography is done. If you need f/2.8 or 1/2000 second in bright sun with flash, and the flash range will reach your subject, then that's one thing. Everything else is something else, think twice about it.
Beware, the HSS flash concept is a rather special situation, frankly, often not what we want. It is NOT fast flash, it merely allows fast shutter speed and wide aperture with fill flash in bright sun, but at reduced power level (about 20% power, 45% range). The purpose of this mode is NOT to stop fast action, which it cannot do. Regular speedlight mode is what stops fast action, when the ambient light can be kept dim so the ambient and shutter don't blur the motion. Any use of HSS indoors seems a bad plan, in general.
But to use HSS, simply set the camera to Auto FP mode (often Nikon menu E1, on most models that offer Auto FP flash mode), which allows setting the shutter speed faster than maximum shutter sync speed with flash. So, we must also do that too (set a faster shutter speed), because this Auto FP mode only kicks in when the shutter speed is actually faster than maximum shutter sync speed (it remains regular speedlight flash mode otherwise). Camera A or P mode normally cannot provide a fast shutter speed in dim light indoors (dim light is not bright enough to meter a fast shutter speed), so camera M mode probably is necessary to set that shutter speed indoors. Except frankly, alarms should go off at the idea of using Auto FP (or HSS) mode indoors (quite counterproductive), because the regular speedlight mode is tremendously faster than the shutter speed, and more powerful then too. But the idea is that the Auto FP option permits a fast shutter speed which can specifically allow fill flash at f/2.8 in bright sunlight (if you crave to do that). The flash power is substantially reduced, so get closer and watch the TTL Ready LED warnings. That's about it, but there is more below that you really ought to know...
Anything but fast, the full opposite, continuous light cannot stop motion at all. All HSS flash has going for it is the shutter speed (which reduces the light, which requires wider aperture to compensate... just like any continuous light).
HSS is only High Speed Sync (just meaning that continuous light has no sync requirements, which allows all faster shutter speeds, same as sunlight does). And HSS is reduced level too, the maximum power level is about 20% of speedlight flash mode power level.
A hand grinding tool is shown, the grinding disk is one inch diameter (25.4 mm). The Black&Decker manual claims 24,000 RPM (400 revolutions/second).
D300 camera (manual mode), ISO 200, with SB-800 (manual flash mode), on camera hot shoe at about 33 inches (0.84 meters). Note the HSS flash is at 1/2 power, even this close (here at 33 inches). The 1/8000 shutter does drastically reduce its continuous power, but the f/3 aperture compensates for that aspect (equivalent exposure). The speedlight mode is set to f/3 too, but there's nothing to compensate for, so it can use 1/128 power.
Regular speedlight mode, at 1/128 power.
f/3 1/250 second. Brighter, but left as is.
Shown stopped, and turning 24,000 RPM.
The shutter speed may be only 1/250 second, but the SB-800 manual says regular flash 1/128 power is 1/41,600 second duration, a speedlight. The shutter stood fully open for an instant at 1/250 second, but the room light was dim, and the flash illumination is only on a very much shorter time (1/41600 second).
HSS flash mode, at 1/2 power.
f/3 1/8000 second.
Shown stopped, and turning 24,000 RPM.
The HSS try cannot stop the motion, even at 1/8000 second shutter speed. Weird distortion effects from the focal plane shutter (ink lines are not shown at actual 90 degrees) because the HSS shutter was a 1/8000 second open slit moving up the frame during about a 1/330 second shutter travel time. HSS is NOT the way to stop motion.
If 400 revolutions per second, the rim speed computes 105 feet/second, or 71 miles/hour, which is pretty fast, especially if seen from only 33 inches. A 1/41600 second speedlight flash computes 3.5 degrees of rotation, so we did not quite stop it. But 1/8000 second HSS shutter computes 18 degrees, except it is not exactly 1/8000 second. The exposure is, but HSS is continuous light and a 1/8000 second shutter slit traveling down the frame (over maybe roughly 1/330 second). The ink lines rotated while the shutter slit was moving down the HSS frame, which distorts where they are shown to be, at different times and places in top and bottom.
HSS IS NOT FAST FLASH. Nor a powerful flash. But HSS does allow f/2.8 in bright sun, at short ranges. This 1/2 power at 33 inches may not be impressive, but a -2 EV fill flash level, and full power and a slower 1/4000 second shutter could make a major difference, maybe 11 feet.
1/1050 sec. at M 1/1 Full output (t.5)
1/1100 sec. at M 1/2 output
1/2700 sec. at M 1/4 output
1/5900 sec. at M 1/8 output
1/10900 sec. at M 1/16 output
1/17800 sec. at M 1/32 output
1/32300 sec. at M 1/64 output
1/41600 sec. at M 1/128 output
1/128 power to 1/2 power is six stops. Real speedlight flash mode is normally a huge advantage, unless you crave to use f/2.8 with fill in bright sun. Or perhaps if in bright sun, sports action really needs a fast shutter speed, but do notice that the above 1/8000 second example is 1/2 power at f/3 at 33 inches (ISO 200). HSS Fill flash at -2 EV would have twice the range of 0 EV, but most outdoor action may be more distant. Think this out, and practice in the back yard before you show up for an important shoot.
Make no mistake, HSS mode flash is NOT fast flash, HSS is the furthest thing from fast. HSS is continuous light, which can rely only on shutter speed to stop motion. The regular speedlight mode is of course a very fast flash (much faster than shutter speed if at lower power levels), and the very major point is that using speedlight flash is the basis of how we achieve high speed photography, like say stopping water drop splashes. See this.
It is Nikons way to enable HSS mode, which is a way to bypass the maximum shutter sync speed limit with flash - mainly to allow fast shutter speed with flash in situations maybe like 1/6400 second f/2.8 in bright sunshine. Auto FP shifts into HSS mode only when and if the actual shutter speed exceeds the Auto FP menu speed.
In the past history of flash bulbs, there was a special longer-burning flash bulb type for focal plane shutters, called FP sync. This FP flash bulb would burn more slowly than others, to provide light for a relatively long time (the overall shutter curtain travel time), so that no matter where the narrow focal plane shutter slit was in its travel across the frame, there was still flash light coming through it. This longer flash duration allowed focal plane shutters to use faster shutter speeds than the 1/60 second sync speed at that time. HSS flash mode is basically today's electronic FP flash bulb, which simply does the same longer burning job.
Today, we have electronic flash, which (excepting HSS) is usually very fast (short duration). Studio flashes are fairly fast, but camera flashes are even more so fast, called speedights. This speed is normally a fantastic motion-stopping property, however, it is also fast enough to show and stop the moving slit in focal plane shutters, causing dark unexposed bands in the frame, unless maximum shutter sync speed is observed (maximum shutter sync speed is the fastest shutter speed that still opens the full frame, a very wide slit to allow instantaneous flash).
So today, this electronic HSS mode is sometimes provided (for special purposes). HSS is Not fancy technology that ought to always be used because it's nifty. Instead the opposite, which would give up too much. HSS only allows certain specific conditions when and if applicable for specific purpose. This HSS mode signals the flash unit to switch modes in a startling drastic way.... Instead of the one full powered instantaneous burst of strong light, the HSS flash becomes a rapid series of weaker pulses, at a very high repeating rate, keeping the flash tube gas continually ionized, so that the flash tube outputs continuous light (like the sun is continuous, or like an incandescent bulb is continuous). HSS is the equivalent of the old slow-burning long-lasting FP flash bulb. Continuous light is always on, as far as the shutter can see, lasting for the full shutter travel time. The camera FP mode triggers this HSS flash mode slightly before the shutter opens, instead of after the shutter opens, so the shutter will see the continuous HSS light.
This graph is from Nikon patent at the US Patent Office, which describes this continuous FP "flash" mode. FP flash mode is not a new feature, it has been in most Nikon speedlight models since the SB-25 in 1992 (there it is called FP High-Speed Sync). It only worked in Manual flash mode then. This 2002 patent is about FP TTL preflash.
The top graph A is regular speedlight flash mode. The intensity rises quickly to a peak P, and then slowly decays away. The full duration of the graph shown might be maybe about 1/350 second. The marked point P/2 is the half intensity point (and regular flash duration specs are measured between the half power points, called t.5). Actual speedlight half power implementation simply abruptly cuts the output off about there, with a straight vertical chop (see more description). This makes the speedlight duration be much faster, typically about 1/1000 second actual for 1/2 power level flash. For lower power, say 1/16 power, it might last only 1/10,000 second duration. At 1/128 power, it may last only 1/40,000 second duration (these are SB-700 specifications). The speedlight flash can be extremely fast, which is the technique used to stop incredible motion... called a speedlight.
The middle graph B is FP flash mode. (FP flash mode and HSS flash mode are two names for the same thing). FP mode can not change the shutter action, nor override the maximum sync speed about when it is open, but instead it changes the action of the physical flash unit - which is no longer a speedlight pulse, no longer requiring shutter sync. Instead it emits a continuous stream of flash pulses at an extremely high rate (many tens of thousands of pulses per second), the effect of which outputs continuous light, for the full duration T2 of the shutter slit travel time. So that no matter where the narrow open slit is at that instant, there is still light (like the old FP flash bulb). This duration is still quick, so humans still perceive it as a flash, but actually it is a longer continuous light relative to the shutter duration.
A few third party HSS flashes have shown banding at the fastest shutter speeds, but normally the HSS flash rate is so high that even a 1/8000 second slit sees several flashes.
If we assume the case when the second curtain starts closing just as the first curtain is fully opened, then the smallest shutter fully open time is near zero (but enough to allow the maximum regular speedlight flash duration to pass). The exposure duration is still the first curtain travel time to open it. This makes Maximum Sync Speed be approximately equal to the curtain travel time. But the continuous HSS flash has to remain illuminated for the travel time of the first curtain, plus the slit width, plus the second curtain close time. Therefore at any shutter speed, HSS flash duration is at least twice longer than the maximum sync speed. The HSS light has to be constant (continuous light) for this full duration, anytime the slit width is somewhere along its travel. This HSS flash will still seem a brief flash to human eyes, but it is continuous light for a duration greatly longer than a regular speedlight flash.
So HSS mode is not like a "flash" at all now, certainly not still a speedlight, but as seen from behind the shutter, is more like a desk lamp (continuous, always on). This is a huge difference:
The idea is that this HSS mode acts like the old longer-burning FP flash bulb (creates a longer flash duration), so that it can use any faster shutter speed, simply because continuous light has no sync requirement.
This mode needs our full attention. Realistically, Auto FP is only useful in bright ambient, and then simple metering changes affecting shutter speeds near the FP threshold (camera A mode, if near maximum sync speed) can switch this flash mode on or off - from regular flash to HSS flash, unexpectedly shortening flash maximum distance range, in one frame to the next. My own notion is that we ought to always know what we are trying to do, and we ought to always know what the camera is going to do, so we ought to turn Auto FP off when we don't want it, and want no risk of it. We will certainly know when we want it, and can turn it back on then.
But anytime the camera menu is in Auto FP mode, then the flash LCD displays "FP" at any shutter speed, regardless if the shutter speed is actually setting HSS mode or not... I think just as a warning to users that the shutter speed can go higher now, allowing us to enter this mode. It would seem much more helpful if the display instead said FP HSS when it was HSS.
Camera P mode is good for regular TTL fill flash in bright sun outdoors - many words can be said (elsewhere here, Part 4), since P mode understands both requirements, for ambient and flash. However, P mode will try its best (with flash) to Not exceed maximum sync speed when Auto FP is enabled. In camera P mode, the lens will stop down to f/32 before it lets shutter speed exceed maximum sync speed. This is like Auto FP is turned off, which in fact is normally better for the speedlight performance. But you can still spin the shutter dial (Flexible mode in P mode) for faster shutter speed to achieve FP mode. I'm not knocking FP mode for what it is or does, but we don't want this FP shift happening unexpectedly, unaware. HSS flash is a very special situation, and we know when we want it, and should turn it on then. FP is pretty much the last resort for P mode, but if the flash is not present, then shutter speed is quickly given more priority.
Camera A mode uses the aperture we set. We need to realize that regular flash mode in bright sun at ISO 200 will need to be about f/16 (for maximum sync speed shutter). Auto FP mode can use any faster shutter speeds, BUT when the resulting shutter speed is near the Auto FP threshold, just moving the camera slightly can change shutter speed slightly to shift into and out of HSS mode, which greatly affects available flash power and range, possibly from shot to shot. Of course, it is true that without Auto FP, camera A mode limits out at maximum shutter sync speed then, and proper exposure still fails (until we set a workable aperture, near f/16 for bright sun and ISO 200). So be aware. Be certain you are doing what you actually want to do. You can always turn Auto FP off. You obviously will know when you want to use fill flash with f/2.8 in the sun, and can turn it back on then.
Regular flash is relatively instantaneous, shorter than, and possibly very much shorter than the shutter duration, therefore flash exposure is independent of shutter speed. The shutter merely needs to be fully open when the flash is triggered. But Auto FP mode in HSS mode is continuous light, at least longer than the shutter duration. Humans might see it as a flash, but it turns on before the shutter opens, and turns off after the shutter closes, so to the shutter, it is exactly the same as any other continuous light. This is a key point. Continuous light is affected by shutter speed - fast shutter speed drastically reduces the FP power, same as it does sunlight. But a wider aperture does compensate it, same as aperture compensates sunlight (creating what we call "equivalent exposures"). Because it is continuous light.
However, the available HSS flash power is substantially reduced (to be able to run continuously), to be less than 1/4 power maximum (about 20%), and the maximum distance range is substantially reduced. Additionally, the HSS range gets smaller with higher shutter speed. Do not confuse HSS mode with regular flash mode - it is no surprise that its short shutter speed decimates continuous light - because it is exactly same effect as sunlight or incandescent light, which are continuous too. So half the exposure time naturally sees half of the continuous light. And the very reason we use HSS (fast shutter speed) decimates its power even more (but wide aperture compensates to be equivalent exposure) - because it is continuous just like sunlight is. The flash is still subject to the inverse square law - illumination still falls off fast with distance. HSS flash just has a weaker starting point now. TTL automation still works, just with less power capability and shorter range. The reduced external flash power is a little less than the internal flash power level: At ISO 100, SB-700 24 mm zoom HSS FX Guide Number 34 (feet). In comparison, the little internal speedlight flash is near Guide Number 39 (very different flash modes however). However, we can of course always use multiple HSS flash units to recover some of the power. Every time we double the number of flashes, we gain one stop power and exposure. But HSS needs about 2.3 stops to recover, which is five flash units. The point is, the power is not a trivial difference.
HSS is definitely NOT High Speed Flash either, it is merely called High Speed Sync - simply and only because continuous light has no sync requirements. Any shutter speed can be used. Like the Sun has no sync requirement either. And continuous light has no motion stopping ability either, so this flash mode is no longer a speedlight. It may allow a high speed shutter to stop motion in daylight, but HSS is fully the opposite of high speed flash. It no longer acts like a flash at all - it is, and acts like continuous light (acts more like aiming a desk lamp for illumination). It is called High Speed Sync (HSS), only meaning continuous light has no sync requirement.
And forget about using Rear Curtain Sync with this HSS mode - it would make no sense with continuous light. In HSS mode, the flash is always triggered just before the shutter opens, and continues to fire continuously for the full shutter travel time.
1/400 second at f/11
1/800 second at f/8
1/1600 second at f/5.6
1/3200 second at f/4
1/6400 second at f/2.8
1/200 second at f/16 normally should be in this list, but it was omitted because 1/200 second would not enable HSS flash mode (very different).
We know about Equivalent Exposures in daylight - increasing shutter speed naturally reduces the exposure of continuous light, but opening aperture (stop for stop) can easily compensate to maintain the same exposure. This is true for continuous light, so HSS flash works the same way as sunshine (in this regard). If we have a situation already adjusted for both proper ambient and HSS fill flash exposures, we can simply run up and down the list of equivalent exposure combinations, and exposure does not change for either sunlight or HSS flash. Regular flash is nothing like that - regular flash exposure is not affected by shutter speed, so aperture is all important. However, for both modes, the Inverse Square Law is still in effect, affecting all flash.
Equivalent exposures work for any continuous light, so are also equivalent for the HSS flash. When using HSS fill flash in bright sun, we can simply select any other equivalent exposure, and both HSS flash and sun stay the same correct exposure. It is the equivalent exposures of continuous light which allow wide apertures with HSS flash in sunlight. Note again, this is NOT remotely like regular speedlight flash mode works. But HSS mode can use fast shutter speeds which can allow widest apertures with flash in bright sun, if you need to blur the background (and if you have enough flash power). The wide aperture in sunlight is really the main purpose of HSS flash.
Saying it again... This is the Big Deal. We do have the concept of setting different manual HSS power levels (full or half or 1/4), but when a level is selected and triggered, the HSS flash simply turns on at that level. HSS becomes a continuous light, on for the duration. It does not matter what the shutter speed is, the HSS flash outputs that same level, continuously. So exactly like Daylight or Incandescent lights (also continuous), a faster shutter speed (say 1/2000 second) sees only a tiny total from it, but a longer shutter speed (say 1/500 second) sees a lot more light (two stops more). And so exactly like Daylight (or any continuous light), we must adjust f/stop in compensation of shutter speed, to make the exposure right (and equivalent). We can hear mystified people claiming their HSS flash is much brighter at relatively slower shutters, when they don't understand that HSS is simply continuous light. In contrast, regular speedlight flash is very different, a very quick pulse much shorter than the shutter opening (the shutter merely has to be open to pass it, called "sync"), so then shutter speed is not a factor for regular speedlight flash exposure.
So yes, HSS mode does allow faster shutter speeds with "flash", but the only realistic goal is to allow wider aperture in the bright ambient light. It is NOT about stopping motion, since HSS flash is continuous light (cannot stop motion), and the regular speedlight flash is much faster than any shutter speed, in the same situation. One theoretical exception, fill flash for fast action in sunlight could be helped with faster shutter speed of course, if the power level is high enough for the distance (fortunately, normal fill only needs about 1/4 the power of a full flash exposure). But HSS flash really is something we really have to want, since its distance range is short. You can increase ISO to gain flash range, but this also gives away the widest apertures then.
Equivalent Exposure concept of HSS, affects both HSS flash and sunlight. D800 and SB-800 at 5.5 feet to near tree, at ISO 400 and 24 mm. Flash is at full manual power level. The Guide Number is GN 72 at reference 1/300 second and ISO 400. f/16 1/320 worked for the ambient alone. So at f/16, HSS GN distance is 72/16 = 4.5 feet. Using a little more distance here was a reduction to become fill level in daylight (-0.6 stop here). Both flash and sunlight have the same results at f/16 1/320 second and at f/4 1/5000 second. Equivalent exposures is a key point about using HSS flash. The HSS Guide Number chart is for this SB-800 flash.
Clarification about HSS Guide Number: HSS GN is different, and easy, but a little tricky. Shutter speed does not affect speedlight GN, but shutter speed of course does greatly affect HSS GN. HSS Guide number is given for one shutter speed. Two stops faster shutter speed is one half HSS Guide Number. This older GN chart says it is for 1/300 second (1/500 second is more common to see today). HSS has Guide Numbers, for the specified conditions stated. Here 24 mm ISO 400 is GN 72 specified at 1/300 second. So for 1/320 f/16, maximum TTL power is range GN72/f16 = 4.5 foot range (because, in this case, the shutter speed matches the stated situation). This tree is a foot farther, so it's down about -0.6 EV here (let's call it fill flash). But this same 4.5 distance is applied to Any Equivalent Exposure, so 1/5000 f/4 is the same 4.5 foot range.
For HSS flash GN, compute HSS range for the specified shutter speed, and then any Equivalent Exposure is equivalent, with same range. Careful though, unlike flash GN, this says 1/300 second, and it does NOT mean 1/5000 f/4 has 72/4 = 18 foot range. It does mean that 1/5000 second is -4 EV from 1/320 second (1/16 exposure), so for 1/5000 second, we open the aperture 4 stops to f/4, with same range of the same Equivalent Exposure.
Of course, that picture has no use for 1/5000 second (it's too small to see detail, but better depth of field is surely much more important). And regardless of the next test (also totally inappropriate use of HSS) it is really hard to imagine any reason to use HSS flash indoors, since we don't worry about the dim ambient, and normal speedlight mode will run circles around it (speed and power and range). The next frames are HSS at full Manual power level in this indoor test at 3.7 feet, merely to show Equivalent Exposure for HSS.
Indoor Example --- Equivalent Exposures in HSS flash mode - ISO 100
It seems counterproductive to use HSS flash indoors (dim ambient), because regular speedlight flash mode runs circles around it, regarding speed and power and range. Note that this maximum HSS range is very short (3.7 feet, about 1.1 meters), but fill flash level at -2 EV would be twice more range. Fill flash would seem the only point of HSS mode.
Sun and Shadow Example - Equivalent Exposures in HSS flash mode - Nikon D300 ISO 200, SB-800 flash TTL BL
(D300 can sync flash at 1/250 second, most models with HSS can.)
TTL BL mode is "balanced", i.e., auto fill flash, reduced automatically (often near -2 EV) in bright sun.
These examples have no motion or depth of field to show (so regular speedlight flash works as well or better), but those properties of course work as always. HSS does allow flash in sunlight at f/2.8. The excitement here is that since Equivalent Exposures works with continuous light, Equivalent Exposure works in the same way for both sunshine and HSS flash. Which is an important concept, both for understanding, and for use. Makes HSS flash easy. Real flash sure ain't that way. The downside of HSS mode is low flash power with no motion stopping ability- but it can allow using flash at f/2.8 outdoors in bright sun.
The top set is indoors, at 3.7 feet, at maximum manual flash power in all frames. Bottom set is outdoors at seven feet. Manual flash compensation was 0 EV, but TTL BL automation is providing about -2 EV (in bright sun cases). While maximum HSS level is a reduced level, automation also adjusted the BL flash level (clearly the same level in all frames).
The HSS mode gave up all of its speedlight speed, but Equivalent Exposure permits a fast shutter speed to allow wide apertures... if that's what you want. The bigger beauty is that both HSS flash and bright sunshine do Equivalent Exposures tracking together (simply because both are continuous lights).
Guide Numbers: We often use TTL fill flash of course, but this tedious next indented section is about how the HSS Guide Number chart works with manual power. It helps us visualize the HSS power capability. The indoor pictures above are from this example.
There is a Guide Number calculator just for HSS flash.
The test above is the quickest quickie, it took less than a minute to shoot six frames. Essentially no planning because both sunlight and HSS provide the same equivalent exposures. The entire idea of the concept is that HSS is constant continuous light, so any fast shutter can be used (there simply is no sync issue in continuous light). And since it is constant power (same manual flash power setting), the flash exposure and range, and the sun's exposure, remain the SAME at Equivalent Exposures (the HSS flash at constant power meters the same at any equivalent exposure).
In the sun, if the sun meters f/11 at 1/250, then any equivalent exposure works for it. If we add TTL BL fill flash, it adjusts power to the situation. If at shutter speeds switching into HSS mode, then any equivalent exposure works for it too.
The big deal for fill flash: Note that Fill flash at two stops down from full exposure would be correct at double this computed GN distance. Greater distances would be down even more than the -2EV, but it still might help the picture a little.
Yes, the HSS flash range is limited power (but it certainly is not zero), and HSS flash has no motion-stopping properties (but you still have shutter speed). You may really have to want this. The HSS flash mode is no longer a speedlight, and HSS mode does seem worst possible choice for indoor lighting (since the speedlight mode is more powerful, and faster than the shutter speed). For example: a similar 1/4 power in SB-800 regular speedlight mode at f/5.6 in this same setting is properly exposed at 14.5 feet range and 1/2700 second flash duration. But yes, HSS mode can allow wide apertures with flash in bright sun (at short range).
So I left 1/200 f/16 off of the first Bright Sun Sunny 16 equivalent exposure list above, because it is not equivalent to the others with HSS flash (not at all like say f/11 1/400 second). 1/200 second is regular flash mode only. Faster than maximum sync speed is HSS flash mode only. The HSS flash mode changes drastically, but any shutter speed not exceeding maximum sync speed (assumed 1/200 second here) will still be regular flash mode. But above maximum sync, the shutter cannot sync regular flash. Regular flash mode is nearly instantaneous (and it stops motion well), and can also achieve full power levels, properties which are normally a plus (all except maximum shutter sync speed).
Indoors, or in dark shade, without the bright sun, it seems wise to forget about using Auto FP flash. In many cases, it would be a serious mistake, because regular flash runs circles around it, regarding power and range and speed. It is the speedlight that is fast, which stops the motion... not the shutter sync speed. Maximum power HSS flash mode is less than 1/4 power. At 1/4 power, speedlight mode is faster than 1/2500 second itself. In a more dim situation, without strong ambient light to blur the motion, the speedlight is often MUCH faster than any shutter speed can be. And about 5x more powerful than HSS mode can be.
We see experiments posted on the internet, showing "Look at me, I can use 1/4000 second shutter speed with flash!" The inexpensive electronic chip shutters can do it, and HSS flash mode can do it. Yes, HSS mode can allow wide aperture, and shutter speed could be useful for motion in bright daylight (if you have the distance range). I do get tickled at the web sites plotting silly charts of how HSS flash power varies with shutter speed. They must not understand that HSS flash mode has become a continuous light, which simply turns on, and stays on, same power regardless of shutter speed, and then the shutter speed does its own thing, doing all of the timing. The Equivalent Exposure concept is much more useful.
But for speed, a SB-700 at regular mode 1/32 power (for close range) flash duration is 1/25,000 second! (spec chart in rear of flash manuals, SB-700 page H-17). Regular flash mode of course, it is called a "speedlight", and speedlight is a "feature", called High Speed Flash Photography, and this speed can tame water drop splashes, bursting balloons, hummingbird wings, and camera shake for macro work. Any shutter speed is a small fraction of that speed. SB-700 1/32 power DX is Guide Number 16 (feet) at ISO 100 and 24 mm zoom, which allows f/16 at one foot. Or ISO x 4 doubles Guide Number, which doubles range. Opening two stops of aperture also doubles range of speedlights. Regular speedlight flash is usually very feasible, in spite of Maximum Shutter Sync Speed, which is no issue in dim light where we need flash.
It seems foolish to ignore the differences. Speedlights are the basis of High Speed Flash Photography. We do have better tools available than some may realize.
The most common wrong attitude to see discussed about Auto FP is this description:
Which is quite correct, but it is limited thinking, and seems not to grasp the idea about using it. Yes, this is quite different than regular speedlight flash, which exposure is independent of shutter speed. But do realize HSS is instead continuous light, and that sunlight and incandescent light work exactly that same way, the same "loss of -1EV for each halving of shutter speed". That's how all continuous light works. We get over it. :)
In bright sun, we know 1/125 second at f/16 works at ISO 100. Change the shutter speed to 1/4000 second, and it becomes 5 stops underexposed. This is how continuous light works... it ought not be any surprise. Just open the aperture to f/2.8 to compensate, and we are back in business. HSS flash mode (FP) is also continuous light, and Equivalent Exposures work the same way for HSS flash as for sunlight or incandescent light.
Since Equivalent Exposures work for continuous light, we simply compensate by opening the aperture, one stop for one of shutter speed. Then we do not see any exposure difference at all (called Equivalent Exposure). Same as any continuous light picture using sunlight or incandescent light. It is only regular speedlight flash does not work that way (speedlight duration is faster than the shutter speed, so speedlight exposure is independent of shutter speed). However, HSS flash is continuous light (for the duration of shutter operation), and it does work the same way as other continuous light. Open aperture one stop for one stop of shutter speed, and we have not lost a thing (except depth of field). It's that initial 80% HSS loss that we miss. :)
Said again, it's important. The HSS flash does change modes to flash continuously (on for the duration of the shutter curtain travel time), which does cut it down to about 20% of the full power level. That is certainly a loss, which does affect us greatly. And it becomes continuous light now, so faster shutter speeds do also cost power. For example changing shutter speed to 1/4000 second from 1/500 second is only open 1/8 of the previous time, which for continuous light, does cost us all but 1/8 of the light we had... if assuming that we leave the aperture where it was. But which would not be the same exposure of sunlight or HSS flash, so leaving the aperture there is not our plan. HSS is continuous light, same as sunlight is continuous, so Equivalent Exposures can easily compensate. If we also open aperture three stops (8x more light), we are back where we were, no loss at all with the faster shutter speed (we do still have the initial mode change however). This ability is our only reason for considering HSS mode, and Equivalent Exposure is the only way to achieve that combination.
So if and when maximum shutter sync speed is a limiting problem in bright sun, maybe consider Auto FP. That's what it's for. This is all that it is for.
More HSS power would be good though. Shutter speed is easily compensated with Equivalent Exposures, but the initial mode loss is serious. Joe McNally promotes HSS mode for shoots outdoors, but his videos show him putting about four SB-900 in one umbrella. :) Four flashes acting as one is two stops more power, double range (almost back up to one speedlight level). More power in the sun is always good.
Continued, some HSS flash Power examples