This article is too long unless you actually want to hear more details. Here is a short version:
HSS flash (Nikon Auto FP mode) is an entirely different flash mode, for focal plane shutters, where the flash is triggered repeatedly and rapidly (tens of thousands of times per second), so that it mimics being "on" like continuous light (like sunlight or a desk lamp is on continuously). As seen by the shutter (flash is on when shutter opens, still on when shutter closes), it is continuous light, like sunlight. Humans still see a "flash" (length of the shutter travel duration), but this continuous "flash" stays on during the longer focal plane shutter travel time. For example, perhaps HSS for 1/200 second (5 ms) instead of 1/16 power speedlight for 1/10,000 second (0.1 ms). To be able to be on continuously 50x longer, it can only run at about 20% power level maximum.
The purpose of the "flash" being made to be continuous is because there is no concept of sync with any continuous light - any shutter speed works (which is the only reason it is called High Speed Sync - there is no sync issue at any fast shutter speed). Like sunlight, it simply works at any shutter speed. Like sunlight, a very fast shutter speed reduces the light, which requires a very wide aperture in compensation. However HSS is NOT high speed flash. HSS no longer works same as "flash", and is no longer a speedlight - continuous light has no motion stopping capability (we do still have the shutter speed, but the speedlight was typically faster).
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 should be in this list, but was omitted because 1/200 second would not enable FP flash mode (very different).
The usual reason to use this FP 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 for fill.
So, while HSS can use any high shutter speed, a fast shutter speed will tremendously diminish the continuous light captured (same as sunlight does). However, since it is continuous, Equivalent Exposures apply again, now applies to FP flash too, same as sunlight, meaning a wide aperture can compensate for the light losses of a fast shutter speed, for both FP flash or for ambient, since both are continuous. To be able to use wide aperture with flash is the usual big deal (for 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 in typical indoor scenes. The regular speedlight mode will run circles around it, power, range, speed. But the focal plane shutter is not fully open to sync speedlights at shutter speeds higher than about 1/200 second (ballpark for many cameras).
In bright sunlight ambient, flash mode TTL BL should provide the proper flash fill level automatically (point&shoot 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 doubles the range of brighter flash seeking 0 EV as main light. And in this HSS FP 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 FP 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 camera and flash communicate about this, a matched system. This system communication (and thus FP HSS mode) is not possible except on the hot shoe or via Commander. Manual or TTL flash is possible then. The Auto FP HSS feature is typically is not offered on entry level flash or camera models.
Note: The Nikon camera internal flash cannot do FP flash, but it can still be the Commander used to trigger FP mode in remote flashes. However, since it cannot do FP 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), so no harm is done by skipping this page for now. Next Page - Soft Light.
Nikon at times says HSS, but they call this feature Auto FP flash (Focal Plane mode). Canon calls it HSS (High Speed Sync), same thing. This FP mode feature allows use of any fast shutter speed with "flash", by drastically changing operation of the flash unit itself, to no longer be an instantaneous pulse, no longer a speedlight, but to instead be a continuous light (for duration of focal plane shutter travel). This FP flash is a feature of higher end system gear.
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 affecting all models with a focal plane shutter). This is not much issue for flash indoors (the speedlight is a lot 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 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 see a 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.
There seems only two conceivable purposes of FP flash mode, both involving bright ambient (sunlight).
Beware, the FP 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 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...
It is a way to bypass the maximum shutter sync speed limit with flash - mainly to allow flash in situations maybe like 1/6400 second f/2.8 in bright sunshine.
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 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 (previous page), there was still flash light coming through it. This longer duration allowed faster shutter speeds than the 1/60 second sync speed at that time. Auto FP mode is basically today's FP flash bulb, which simply does the same longer burning job.
Today, we have electronic flash, which is usually extremely fast (short duration). Camera flashes are even more so, called speedights, but studio lights are fairly fast too. This speed is normally a fantastic motion-stopping property, but it is fast enough to show the slit in focal plane shutters, causing dark unexposed bands in the frame, unless maximum shutter sync speed is observed.
So today, this electronic Auto FP mode is sometimes provided (for special purposes). This mode signals the flash unit to switch modes in a startling drastic way.... Instead of the one full powered instantaneous burst, the FP flash becomes a 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). This 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 flash mode slightly before the shutter opens (during the shutter lag time), instead of after the shutter opens, so the shutter will see the continuous FP light.
This graph is from a 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 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 a regular speedlight flash at full power, the intensity rises quickly to a peak P, and then slowly decays away. The full duration of the graph shown might be maybe about 1/300 second. The marked point P/2 is the half intensity point. Actual speedlight half power implementation simply abruptly cuts the output off about there, with a straight vertical chop (see more description). This makes the duration be much faster, typically about 1/1000 second. 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 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 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. Instead it emits a continuous stream of flash pulses at an extremely high rate (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 is still quick, so humans still perceive it as a flash, but it is a longer continuous light relative to the shutter duration.
If we assume that the smallest fully open time is near zero (but enough to allow the maximum regular speedlight flash duration to pass), its exposure duration is still the first curtain travel time to open it (making Maximum Sync Speed be only very slightly longer than curtain travel time). But the continuous FP flash has to remain illuminated for the travel time of the first curtain, plus the slit width, plus the second curtain close time. The light has to be continuous and constant for this full duration, anytime the slit width is somewhere along its travel. This FP flash (shutter duration) may still seem a brief flash to human eyes, but it is greatly longer than a regular speedlight flash.
So FP 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 FP 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 FP 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.
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. It 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) to achieve FP mode. I'm not knocking FP mode for what it is or does, but we do not want this FP shift happening unexpectedly, unaware. FP 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). FP mode can use any faster shutter speeds, BUT when the resulting shutter speed is near the FP threshold, just moving the camera slightly can change shutter speed slightly to shift into and out of FP 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 duration 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 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 FP 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 FP range gets smaller with higher shutter speed. Do not confuse FP 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 FP (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. FP 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 FP FX Guide Number 34 (feet). In comparison, the little internal flash is about Guide Number 39 (very different flash modes however). However, we can of course always use multiple FP flash units to recover some of the power. Every time we double the number of flashes, we gain one stop power and exposure. But FP 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 because continuous light has no sync requirements. 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 FP mode - it would make no sense with continuous light. In FP 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 should be in this list, but was omitted because 1/200 second would not enable FP flash mode (very different).
We know about Equivalent Exposures in daylight - increasing shutter speed naturally reduces the continuous light seen, but opening aperture (stop for stop) can compensate to maintain the same exposure. This is true for continuous light, so FP flash works the same way. If we have a situation already adjusted for both proper ambient and FP fill flash exposures, we can can simply run up and down the list of equivalent exposure combinations, and exposure does not change for either sunlight or FP 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 FP flash. When using FP fill flash in bright sun, we can simply select any other equivalent exposure, and both FP flash and sun stay the same correct exposure. It is the equivalent exposures of continuous light which allow wide apertures with FP flash in sunlight. Note again, this is NOT remotely like regular speedlight flash mode works. But FP 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 FP flash.
Saying it again... This is the Big Deal. We do have the concept of setting different FP power levels (full or half or 1/4), but when a level is selected (automatic with TTL) and triggered, the FP flash simply turns on at that level. FP becomes a continuous light, on for the duration. It does not matter what the shutter speed is, the FP 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 FP flash is much brighter at relatively slower shutters, when they don't understand that FP 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, FP mode does allow faster shutter speeds with "flash", but the only realistic goal is to to allow wider aperture in the bright ambient light. It is NOT about stopping motion, since FP 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 FP 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 costs the widest apertures then.
Regardless of the next test, it is really hard to imagine any reason for using FP flash indoors, since normal speedlight mode will run circles around it (speed and power and range). All frames are full manual power level in this indoor test at 3.7 feet, to make a case for Equivalent Exposure.
Indoor Example --- Equivalent Exposures in FP flash mode - ISO 100
There seems no advantage of using FP flash indoors (dim ambient), because regular flash runs circles around it, regarding speed and power and range.
Sun and Shadow Example - Equivalent Exposures in FP 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 FP flash. Which is an important concept, both for understanding, and for use. Makes FP flash easy. Real flash sure ain't that way. The downside of FP 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 FP level is a reduced level, automation also adjusted the BL flash level (clearly the same level in all frames).
The FP 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 FP 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 FP Guide Number chart works with manual power. It helps us visualize the FP power capability. The indoor pictures above are from this example.
Example: SB-800, ISO 100, zoom 70 mm, full manual power, SB-800 chart says FP Guide Number is 59 (feet).
So 59 feet / f16 = 3.7 feet distance (I placed the camera and flash at a measured 44 inches).
So at that distance, that is f/16 at 1/300 second, or any other equivalent exposure. Except, if shutter speed drops to FP threshold, Auto FP switches back to speedlight mode, and it gets brighter, which is the regular GN chart then.
SB-700 (page H-26) DX is GN 55.8, /f16 = 3.49 feet, 42 inches (these are at 1/500 second)
SB-700 (page H-26) FX is GN 51.2, /f16 = 3.20 feet, 38 inches
The test above is the quickest quickie, it took less than a minute to plan and shoot six frames. The entire idea of the concept is that FP is constant continuous light, so any fast shutter can be used (there 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 FP 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 FP 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 FP flash range is limited power (but it certainly is not zero), and FP flash has no motion-stopping properties (but you still have shutter speed). You may really have to want this. The FP flash mode is no longer a speedlight, and FP 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, FP 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 FP 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 FP flash mode only. The FP 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 FP 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 FP mode can be.
We see experiments posted on the internet, showing "Look here, I can use 1/4000 second shutter speed with flash!" The inexpensive electrical chip shutters can do it, and FP flash mode can do it. Yes, FP 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 FP flash power varies with shutter speed. They must not understand that FP 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 it 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 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. Doubling ISO doubles range each time doubled (ISO 800 is 8x range). Opening two stops also doubles range. Regular speedlight flash is usually very feasible, in spite of Maximum Shutter Sync Speed.
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". 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, and we are back in business. HSS flash mode (FP) is also continuous light, and works the same way.
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, FP 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% FP loss that we miss. :)
Said again, it's important. The FP 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 FP flash, so leaving the aperture there is not our plan. FP 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 FP 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 FP power would be good though. Shutter speed is easily compensated with Equivalent Exposures, but the initial mode loss is serious. Joe McNally promotes FP 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.
The full opposite, continuous light cannot stop motion at all. All HSS flash has going for it is the shutter speed.
A hand grinding tool is shown, the grinding disk is one inch diameter. The Black&Decker manual claims 24,000 RPM (400 revolutions/second).
D300, (manual mode), ISO 200, with SB-800 (manual flash mode), on camera hot shoe at about 33 inches. Note the FP flash is at 1/2 power, even at only 33 inches. The 1/8000 shutter does drastically reduce its continuous power, but the f/3 aperture compensates for that aspect (equivalent exposure).
Regular speedlight mode, at 1/128 power.
f/3 1/250 second. Brighter, but I left it alone.
Top stopped, and bottom 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 was fully open 1/250 second, but the room light was dim, and the flash illumination is only on a very short time (1/41600 second).
FP flash mode, at 1/2 power.
f/3 1/8000 second.
Top stopped, and bottom turning 24,000 RPM.
The FP frame does not even stop the center screw head well. Weird distortion effects from the focal plane shutter (ink lines are not shown at 90 degrees) because the FP shutter was a 1/8000 second slit moving up the frame during a longer travel time.
If 400 revolutions per second, the rim speed computes 105 feet/second, or 71 miles/hour, which is pretty fast if seen from 33 inches. A 1/41600 second speedlight flash computes 3.5 degrees of rotation. And 1/8000 second FP shutter computes 18 degrees, except it is not exactly 1/8000 second. The exposure is, but FP is continuous light and a 1/8000 second shutter slit traveling down the frame (over maybe roughly 1/300 second). The inked lines rotated while the shutter slit was moving down the FP frame, which distorts where they are shown to be, at different places in top and bottom.
1/128 power to 1/2 power is six stops. FP mode accounts for some of it, and of course, 1/8000 to 1/500 second is five stops for continuous light. Real 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, 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). Fill flash at -2 EV would have twice the range of 0 EV, but that would be only 66 inches here. 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, FP mode flash is NOT fast flash, it is the furthest thing from fast. It 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 maybe this.
FP flash has advantage of allowing flash fill with a fast shutter and a wide aperture in bright sun. This is its purpose. Its purpose is NOT speed, continuous light has no speed capability at all.
FP flash also as a disadvantage. It is continuous light at much lower power level. It is also decimated by fast shutter speed (like sunlight), but aperture can compensate that with Equivalent Exposures. But continuous light has no motion stopping ability like flash does, just like sunlight in that regard. And basically, its maximum power level is around 2.3 stops less power than regular flash mode.
Is it actually 2.3 stops loss? Here are four ways to verify it.
1. The Nikon SB-700 (page H-25, H-26) and SB-910 (page H-20) prints Guide Numbers for FP mode. FP mode reduces GN by a factor of 2.18x (for 1/500 second shutter), which computes to be 2.24 stops loss. Continuous light exposure of course suffers greatly at fast shutter speeds, however if aperture is also compensated to create equivalent exposures, then this is a constant ratio (i.e., the FP GN for 1/500 second shutter speed is applicable to all equivalent exposures, at any shutter speed).
Guide Numbers from SB-700 user manual, 24 mm zoom ISO 100, Standard pattern, DX mode
Page H-26, Auto FP HSS, DX Guide Number 42 (f/4.2 at ten feet)
Five FP units combined as one, is GN 42 x sqrt(5) = GN 93.9 (f/9.4 at ten feet)
Page H-24, Regular flash mode, DX Guide Number 91.9 (f/9.2 at ten feet), which is only 0.06 stops less than five FP, and 2.3 stops more than one FP.
2. Setup a regular TTL flash on a tripod, with the distance slightly too far, such that the flash LCD shows you a -1/3 EV underexposure warning at maximum shutter sync speed (perhaps via f/16 and/or bounce). Then increase shutter speed 1/3 stop to enable FP mode and take another. Now the SB-800 shows a -2 2/3 stop warning, which is -2.3 EV difference (this warning can only go to -3EV).
3. Take a test picture on a tripod, at maximum sync speed, and also 1/3 stop faster shutter to trigger FP mode, and adjust aperture to give equal exposure to match. This is a crude "measurement", but my SB-800 results show -2.3 stops. It is this ballpark. Result is something less than 1/4 power, maximum.
4. SB-700, SB-800, SB-900, SB-910 rear LCD shows (when the flash head is level straight ahead) the maximum flash range for whatever settings are in effect. The SB-800 flash LCD shows the following full power maximum ranges, using settings of 50 mm zoom, ISO 200 at f/4 and D300 camera 1/250 Auto FP option (f/4 is an arbitrary fixed constant here - for a number). Flash Compensation is zero.
52 feet - For ANY shutter speed not exceeding maximum shutter sync speed (full power)
22 feet - 1/320 second (continuous FP mode kicks in above 1/250 second - reduced power)
20 feet - 1/400 second
14 feet - 1/800 second
10 feet - 1/1600 second
7.0 feet - 1/3200 second (shutter speed decimates continuous light)
4.9 feet - 1/6400 second
Again, all at f/4, which are NOT equivalent exposures (but all FP equivalent exposures are equal range).
Inverse Square Law says 1.414x distance is one stop falloff.
But at the FP shift point above, the ratio of 52 feet changing to 22 feet on the SB-800 LCD computes 2.48 stops loss (some round off in the numbers shown). Note that FP mode works same as continuous sunlight - each faster shutter stop is half as bright (Unless also equivalently compensated by a one stop aperture change). For flash, double power is 1.4x distance range. 14 feet is one stop more than 10 feet, and 20 feet is one stop more than 14 feet. Very important to realize that we did not compensate by opening aperture one stop each step, to create the customary equivalent exposures we deal with continuous light. People are surprised that the FP flash falls off this way with shutter speed, because they are used to regular flash which is not affected by shutter speed. But FP HSS is not regular flash, and the sun and other continuous lights are decimated the same by shutter speed, in exactly the same way, so no reason to get excited. :)
A second set of numbers, at ISO 800 and 105 mm zoom, and same 1/250 second Auto FP mode at f/4:
The first value is limited in the display at 66+ feet. But this is from Guide Number, which for ISO 800, computes GN 184 x 2.8 / f4 = 129 feet.
66+ feet (which is 129 feet) - For ANY shutter speed not exceeding maximum shutter sync speed (full power)
56 feet - 1/320 second (continuous FP mode kicks in above 1/250 second - reduced power)
50 feet - 1/400 second
35 feet - 1/800 second
25 feet - 1/1600 second
18 feet - 1/3200 second (shutter speed decimates continuous light)
12 feet - 1/6400 second
Again, all at f/4, which are NOT equivalent exposures (but all FP equivalent exposures are equal range).
A -1 EV stop of Flash Compensation (for fill) increases distance range by 41%, in either regular or FP mode.
These methods all introduce an additional 1/3 stop shutter speed increase, as the only way to enable FP mode, which are not quite equal situations. But 2.3 stops is the ballpark loss, about 20% power level.
Pictures below show a garage door, looking west at 11:20 AM, in partial shade from a roof shadow at top, and a tree shadow lower. Unfortunately some minor clouds, some minor variance, but I tried. The fill flash is illuminating the dark shadow on the garage door of course (concept works same as a dark shadow on a human face). Nikon D300 in 1/250 second Auto FP mode with hot shoe SB-800. ISO 200 and Center metering, Aperture priority. 24-70 mm lens at 24 mm. Subject distance (garage door) was carefully measured to be at 12 feet (3.66 meters), which is about the limit for SB-800 FP fill flash to help much.
So, is FP HSS fill flash usable in bright sun? Yes, for the purpose of a wider aperture. Is it powerful? No. And the regular speedlight is not so strong either (not at the necessary f/16). So perhaps FP mode may not be optimum power for fill at 12 feet, but we still get considerable helpful flash fill, often usable for f/2.8 in bright sun if desired (within these range limits).
Note: I gotta say, generally, the ONLY goal of any of this FP flash business is just to be able to use fill flash in bright sun, at wide apertures like f/2.8, if we crave that. Otherwise, FP flash is rather weak and its range is limited. It is the full opposite of a fast flash. It conceivably could allow flash with fast shutter for fast action in sun, but the range may be too short for action. IMO, we'd be dumb to use FP mode indoors, where regular flash will run circles around it. But... FP can allow fast shutter so we can use f/2.8 in bright sun.
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