Nikon HSS Auto FP flash mode
Not really a flash fundamental at all, very special purpose, but focal plane shutter cameras often offer a FP High Speed Sync mode (often named Auto FP), which in fact does allow shutter speed faster than the maximum sync speed possible. BTW, the terms FP High Speed Sync, or HSS, or Auto FP, are all the same in my vocabulary here. HSS is the flash feature, and the Nikon CLS feature of Auto FP is the camera option that switches it on for flash.
The internal flash unit cannot do HSS Auto FP mode... The SB-600/SB-800/SB-900 can, on hot-shoe or remote via Commander. But if the camera internal flash is to be used as the commander for FP mode, it must be disabled itself (builtin flash mode set to "- -" in the commander menu), and then shutter speeds faster than maximum sync speed can be set with FP flash.
This FP HSS mode does not modify the shutter action or the maximum sync speed, but instead it changes the physical flash unit, from being a "flash" to instead emit a continuous stream of flash pulses which mimic continuous light, continuously lasting for the duration of the shutter slit travel time (around 1/250 second for any fast shutter speed). The focal plane shutter is still the same moving slit, but no matter where the narrow shutter slit is in its travel over the frame, the continuous flash is still outputting light then, so no dark bands occur. The flash has literally become a continuous light source, always on with respect to the shutter duration, now same as the sun or an incandescent lamp, now on all the time during the exposure duration. However, the flash unit only has finite power to do this, so this continuous mode necessarily reduces maximum flash power to less than 1/4 power, reduced by at least two stops (which can only compete with sun at close distances, so possibly is insufficient power for some purposes then). But, this FP mode will allow a faster shutter speed with flash, which is used to achieve a wider aperture for fill flash in sunshine.
Anytime Auto FP mode is selected in the camera, the hot-shoe flash LCD mode adds FP to the indicated flash mode, for example, it shows TTL BL FP. But note that this HSS mode only actually becomes active (flash mode changes) ONLY when the shutter speed is actually faster than the maximum shutter sync speed. If shutter speed is not faster than maximum sync speed, then regular flash mode is still in play, because it is better in all respects except absolute shutter speed.
HSS is definitely NOT High Speed Flash, it is merely high speed Sync (simply because continuous light has no sync issues.) It still looks like a flash to the human eye, but the HSS flash we see has much longer duration, long enough to be continuously on for the duration of the focal plane shutter travel, which is a duration about the same as maximum sync speed... even at 1/4000 second shutter speed. HSS is fully the opposite of High Speed Flash. The shutter sees HSS as a continuous light, like the sun, with no motion stopping ability at all. In contrast, a SB-600 in real flash mode at 1/2 power level has a flash duration of only 1/1600 second (page 88, rear spec chart), and is greatly faster yet at lower power levels.
The Nikon SB-800 and SB-900 speedlight flash shows a distance range on its LCD (TTL Mode, flash head pointed straight ahead) which shows the maximum distance at the current settings, for the FP High Speed Sync option too.
Tricky: There are two descriptions here. My emphasis is NOT on this first part, but this first part is what you hear at most other places. This first part is camera Manual mode with a fixed aperture (f/4 here), as if indoors in insignificant ambient, when not concerned about matching ambient exposure ... just for the dramatics of it. :) Note that we would be crazy to use FP HSS flash mode indoors this way (of course use regular flash mode indoors, because it is vastly faster, and much more power).
The SB-800 flash LCD shows the following maximum ranges, using settings of 50mm zoom, ISO 200 at f/4 and camera 1/250 Auto FP option, reported below: (f/4 is an arbitrary fixed constant here)
52 feet - ANY shutter speed not exceeding D300 1/250 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 (still SAME flash power, but shutter speed decimates continuous light)
4.9 feet - 1/6400 second
True enough, but this approach seriously misguides our thinking. It seems terrible at first glance, but we really ought not to react with horror that 1/6400 shutter decimates the power of this light when at the same fixed aperture. Yes, sure, it does this, but so does sunlight, or any indoor continuous lights, which work EXACTLY the same way (continuous), and produce exactly the same kind of reduced numbers (with fixed aperture). 1/6400 second shutter always sees only 1/16 of the continuous light that 1/400 second sees (four stops down). What else did you expect? :)
It is true that instantaneous flash is not affected by shutter speed, but HSS is not flash anymore, this is a continuous light now. HSS appears to flash like flash, but it is continuous for the entire longer duration of the shutter travel time, allowing this flash with any fast shutter speed. For a fast shutter in sunlight, we would open the aperture to exactly compensate the light called equivalent exposure. FP HSS is the same as daylight in that regard. FP is not like real flash anymore. It is continuous, just like sunlight (the difference is that we do not notice sunlight being affected by the inverse square law.) There is an initial power loss, yes, but there is no further effect of faster shutter speed - not when at equivalent exposures.
You can watch your flash LCD scale, and this is what it says as you change shutter speed (again, always at fixed aperture here). Photo tests confirm there is about 2.3 stops of light loss initially (when the shutter exceeds maximum sync speed and the flash converts to continuous mode). And then each two times that shutter speed is doubled gives half the distance at the maximum available flash power again, which is the inverse square law. The flash power is not changing, simply the shutter duration seeing it is changing, but that is effectively weaker. So everything makes reasonable sense, so long as we realize Auto FP flash mode becomes Continuous light, and is subject to the rules of continuous light now (no longer acts like a real flash).
I doubt we would ever consider using Auto FP flash mode this way indoors, FP HSS is for fill in bright sun, to use an aperture wider than f/16. So this description above, while correct, is very misleading. It misguides our thinking down the wrong road, confusing us, and preventing our realizing the big picture. Yes, shutter speed affects the FP flash now. Yes, regular flash is unaffected by shutter speed, however, the problem is that the shutter cannot sync flash faster than maximum shutter sync speed. So FP HSS mode converts the flash to be a continuous light, and then shutter sync speed is no longer any issue. Then yes, a shutter speed of 4 EV faster reduces the light by 4 EV. Said that way, yes, it can sound like a terrible problem, but it is still exactly the same as sunshine works, same as all continuous light works, same as we learned exposure works.
So we would never use FP HSS mode that way (not as sole light source with a fixed aperture) - if we want speed, we would use regular flash then. We use FP HSS flash for fill light in bright sun. Remember, regular flash normally has big advantage over FP HSS mode (power and speed), until the maximum shutter sync speed is reached. This is little issue indoors in dimmer light (the flash is much faster than any shutter, and is unaffected by shutter speed). But outdoors in bright sunshine, the sunlight requires Sunny 16, which is ballpark f/16 at maximum shutter sync speed (at ISO 200). In bright sunlight, the maximum shutter sync speed limit locks us in near f/16 (which also requires a lot of regular flash power). But FP HSS mode was invented for this bright sun situation. It allows using fast shutter to achieve large aperture and minimal depth of field, if we want it (but yes, the power is limited).
So here's the HSS deal
Outdoors in bright sunlight is where we might use Auto FP mode and HSS for fill flash. The emphasis is here, in this section. This is how you would use it. The problem with regular flash mode is that Sunny 16 says ISO 200 in full sun daylight will be exposure f/16 at 1/200 second. We are unable to reselect equivalent exposures of f/11 at 1/400, or f/8 at 1/800, because the maximum shutter sync speed for flash is limited to ballpark of 1/200 second. But Auto FP mode can be used to allow a faster shutter speed, for the purpose of a wider aperture. When we do, we obviously must keep the ambient bright sunshine exposure correct too. So in sunshine, when using camera A, P, or S mode, the aperture and shutter speed will automatically track together and compensate each other, so that the EV exposure always remains the same (the ordinary way exposure works in continuous light).
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
These are all the same equivalent EV exposure, all equal for continuous light like sunshine, or for Auto FP flash which is continuous too. The HSS flash power level does not care which equivalent you use, because then aperture compensates for shutter speed, like it always does, like this chart shows. This is how HSS FP flash should be contemplated, as continuous light. Each of these equivalent exposures use the same power of the sun, and the same power of the flash. The 1/6400 second is no actual additional light loss, because the f/2.8 fully compensates for it (did I mention, just like sunlight?). This equivalence is how exposure has always worked.
We use FP HSS flash for fill light in bright sun. In that bright sun, we can simply open the aperture 4 stops, with a 4 stop faster shutter speed to compensate, to achieve equal EV exposure again, exactly like sunshine has always worked. Flash power is unchanged, just like the sun's power is unchanged, both are constant power for the overall shutter travel time. FP HSS mode is used for fill flash in bright sun, and the sun itself requires the equivalent exposure too. This point being that the FP flash is now continuous light, like sunshine is continuous. It shows flash rules no longer apply to FP HSS mode, continuous light rules apply now.
However those fast shutter speed numbers are simply not possible with regular flash mode in bright sunshine, because of the limitation of the maximum shutter sync speed. But in FP mode, they are possible. There is still the initial 2.3 stops loss with FP, but no additional light loss with shutter speed - not while aperture is compensating with shutter speed for equivalent EV (speaking of FP mode, which is continuous light). The flash LCD will show the FP maximum distance range is not changing with shutter speed now - however it probably only has about 5 to 7 feet of range too (fill flash should be one stop or more down, so 10 to 12 feet ought to work for fill). You could play Joe McNally, and use four SB-900 in parallel. :) Using four flashes gives 2 stops more light than one flash (and 4x more range).
NOTE: As the only light source (indoors), that LCD distance is the range. You always want to use regular flash mode indoors, as it is both powerful and fast, and maximum shutter sync speed is not an issue in the dim indoor light. But the fill flash range is greater than the LCD says, since as fill, it is not the only light source. We are in bright sun, which provides most of the light. The flash is fill, which is normally intentionally underexposed a stop or two. So if you need f/2.8 or 1/6400 second with fill flash outdoors, then on the theory that any added fill is good, the LCD showing 6 feet probably works well for fill flash at up to double that range. WATCH YOUR READY LED warning of insufficient flash power. Changing ISO will shift the EV combinations, but ISO will not change the FP flash range from the continuous light (because TTL flash readjusts flash power for ISO, back to the same intensity and same range).
FP HSS flash is NOT like regular flash. It takes much flash power to match the sun, difficult in any flash mode. Any subject distance of several feet can be a big issue at sunlight exposure levels. The best chance is with a larger flash and staying within maximum shutter sync speed, however Sunny 16 and maximum shutter sync speed says you will be near f/16 then (at ISO 200). The case for real flash mode is this: A wider aperture makes the flash power more effective (wider aperture requires less flash power), because real flash is not affected or limited by the corresponding shorter shutter speed. When shutter speed does not exceed maximum sync speed (real flash mode), we see the flash LCD scale showing 41% greater flash range from every wider aperture stop (equivalent of double power), even while the continuous sunlight exposure stays constant due to the equivalent aperture/shutter EV combinations. FP flash mode does not share that aperture advantage, because FP is continuous light now. The only problem with real flash mode is that in bright sunshine, Sunny 16 and the maximum shutter sync speed will force that aperture to be about f/16 for this very bright EV.
(off topic) A Neutral Density filter (ND), or a lower ISO too, can help to widen that aperture, but they require more flash power too, so the only change then is depth of field, and NOT effective flash power or change of flash range. ND and lower ISO do cause a wider aperture, but both also reduce the flash intensity (whereas shutter speed does not). Of course, we can turn the flash power back up (TTL or manual flash, assuming sufficient flash power exists), but the sunshine does not get turned up, so ND or lower ISO can increase the ratio of flash to sun (turning flash up makes fill brighter), and also depth of field is less at wider aperture, but the flash range is not changed by ND or ISO (we readjust flash power to keep it equal). The ND filter can cause darker viewfinder and AF inability to focus. The point is, neither helps the low FP power situation either (because we need to turn the speedlight power level up to compensate them).
So the case of bright sunlight is when we might use Auto FP to bypass sync speed limits. While the first example is true with aperture fixed at f/4, indoors may not be a practical use. In a dim situation where ambient is insignificant and can be ignored, we would be crazy to consider continuous Auto FP mode, not just because of flash power, not just because maximum sync speed is likely not any issue, but because the regular non-FP speedlight real flash mode makes the flash duration be so much faster than any shutter speed can ever be (around 1/11,000 second at 1/16 power, from spec chart in rear of flash manual). High speed photography is done that way. Auto FP mode would be very counterproductive then. Auto FP is instead for sunshine.
FP flash mode is still automatic exposure in TTL mode, within the limits of its range capability. At a fixed shutter speed, say 1/1000 second (manual mode, fixed and constant, no longer tracking and compensating again), you can change the aperture from say f/4 to f/5.6 to f/8 to f/11, and the flash power level still adjusts to give you exactly the same picture exposure - if - the subject distance is still within the maximum distance range for all apertures, at the bright EV value at which it must compete. This last part is not that easy at these reduced power levels.
FP High Speed Sync mode converts the flash to be continuous light, and then the shutter speed affects it the same as any other continuous light. So note that FP High Speed Sync mode is NOT to be confused with flash. It technically is simply no longer flash. It is continuous light from the flash tube, and the rules of continuous light apply. It also is certainly NOT high speed flash. The flash is continuous, the slowest possible. It is only high speed sync.
HSS flash has advantage of allowing a fast shutter and a wide aperture in bright sun. It has two disadvantages. It is continuous light, and so it is affected by shutter speed, and it has no motion stopping ability like flash does. And basically, it is around -2.3 stops less power than regular flash.
Is it actually 2.3 stops loss? Here are four ways to verify it.
1. The Nikon SB-900 prints Guide Numbers for FP mode (manual page F-20), and FP mode reduces GN by a factor of 2.18x of guide number (for 1/500 second shutter), which computes to be 2.24 stops loss.
2. The ratio of 52/22 feet on the SB-800 LCD above (round off errors) computes 2.48 stops loss (ISO 200, f4, 50mm zoom).
3. Setup a non-FP TTL flash with the distance such that you get a -1/3 EV underexposure warning at maximimum shutter sync speed (perhaps via 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 only goes to -3EV).
4. Take a test picture at maximum sync speed, and also 1/3 stop faster to trigger FP mode, and adjust aperture to give equal exposure to match. My SB-800 results show -2.3 stops, others say they think -2.6 stops. It is this ballpark. Result is something less than 1/4 power, maximum.
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 ballpark.
Some FP Flash examples
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. Nikon D300 in 1/250 second Auto FP mode with hot shoe SB-800. ISO 200 and Center metering, Aperture priority. 24-70mm lens at 24mm. Subject distance (garage door) was carefully measured to be at 12 feet (3.66 meters), which is about the limit for FP fill flash to help.
Garage door in shade, at 12 feet, ISO 200, hot shoe flash
Above: No flash. 1/250 second f/16 (dark shadows are the problem).
Above: Regular TTL flash 1/250 second f/16. The flash LCD range says 8.8 feet. If we had been at 8.8 feet, the flash power would have lighted the shadow to full expected exposure.
At 12 feet, it is fill (40% farther is about -1EV), and we still have shadow, but a ligher shadow.
SB-800 Guide Number at 24mm is 98x1.414 = 138 ISO 200, divided by f/16 is 8.7 feet range (assuming the sole light souce, but the sun is helping considerably here).
Maximum shutter sync speed is 1/250 second with flash, so 1/400 at f/11 is simply not possible (with flash in sunlight).
Above: Regular TTL flash -1 EV flash compensation, 1/200 second f/16. The LCD range says 12 feet
Above: TTL FP HSS flash, 1/400 f/11, 0EV flash compensation
Above: TTL FP HSS flash, 1/6400 f/2.8, 0EV flash compensation. The LCD range says 4.6 feet, yet the flash seems helpful for fill at 12 feet. Note that all equivalent exposures show the same flash range in FP HSS flash mode (works same as sun). It is just not much range. But fill does not not need as much power as a sole light source would need. And note that the regular flash mode LCD above reported only 8.7 feet range, and was still usable (at f/16, which allowed maximum sync speed to be honored).
Above: TTL FP HSS flash, -1 EV flash compensation, 1/6400 f/2.8, The LCD range says 6.6 feet.
This is TTL FP mode, and it is NOT showing warning about exceeding maximum power, so the TTL metering must think it is accurate for -1 EV fill.
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 HSS is not 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).
Above: Again, no flash. 1/250 second f/16. Notice the bricks in upper right.
