Nikon Auto FP flash mode - What is it?

This topic is more special purpose, so no harm is done by skipping this page for now. Next Page - Soft Light.

Nikon calls 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, by drastically changing operation of the flash unit, to no longer be a "speedlight flash", to instead be continuous light (described below).

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 true of all models with 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 do offer an Auto FP - or High Speed Sync mode (HSS), which does allow any faster shutter speed with flash - faster than the maximum sync speed possible - a radically different mode, but with reduced distance range. This is a very special flash feature, and the Nikon CLS feature of Auto FP is the camera option that switches it on.

Beware, the FP flash concept is a rather special situation, frankly, often not what you want. It does allow fast shutter speed and wide aperture with fill flash in bright sun, but at reduced distance range. The purpose of this mode is NOT to stop fast action. 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. There are only two conceivable advantages of FP flash mode. Wide aperture is the usual goal, for reduced depth of field with fill flash, when you must have f/2.8 in bright sun, with flash. Or, the fast shutter speed could help stop motion which bright ambient would blur at sync shutter speed, but frankly, the action may be too distant for the reduced range. But FP flash is the full opposite of fast flash, with no motion stopping abilities at all, and in any not-too-bright ambient, the regular speedlight runs circles around FP mode.

But to use it, simply set the camera to Auto FP mode (often Nikon menu E1, on most models offering FP flash mode), which allows setting the shutter speed faster than maximum shutter sync speed with flash. So, you must do that too, 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 will be necessary to set that shutter speed indoors. Except frankly, Auto FP (or HSS) mode is never what you want indoors (counterproductive), because the speedlight is much faster than the shutter speed, and more powerful then too. But, the Auto FP option permits a fast shutter speed which can allow fill flash at f/2.8 in bright sunlight. The flash power is substantially reduced, so get closer and watch your TTL Ready LED warnings. That's about it, but there is more below that you really ought to know...

In Nikon, the higher end camera models offer Auto FP (often a menu around E1). The camera internal flash cannot do Auto FP mode, nor can the SB-400 flash. But the speedlights can, on either the hot-shoe, or as a remote flash via Commander. Commander mode may still need FV Lock to prevent subject blinking, and typically all these features are bundled together.

Note: The Nikon camera internal flash cannot do FP flash. It can still be the Commander used to trigger FP mode in remote flashes, but since it cannot do FP itself, its own light contribution must be disabled (Built-in flash group Mode set to "- -" in the commander menu). Only when the internal flash is disabled (in Commander menu, or when the internal flash door is shut), only then shutter speeds faster than maximum sync speed can be set to enable FP flash. The internal flash commander must still flash commands when disabled as flash, 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.

Note: Studio lights cannot do FP flash mode, it has to be a system thing. FP flash mode does NOT change the way the shutter works, instead it changes the way the system's flash works.

What is Auto FP flash mode?

It is a way to bypass the maximum shutter sync speed limit with flash - mainly to allow flash with ISO 200 at like 1/6400 second f/2.8 in bright sunshine.

In the old days 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, 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 allowed faster shutter speeds than the 1/60 second sync speed at that time.

Today, we have electronic flash, which is usually extremely fast. Camera lights are called speedights, but studio lights are fairly fast too. This speed is normally a fantastic 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 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, instead of after the shutter opens, so the shutter sees continuous light.

The graph at right is from a Nikon patent at the US Patent Office, which describes this continuous FP "flash" mode, which is radically different than regular "flash". The top graph A is regular flash, the intensity rises quickly to a peak P, and then slowly trails away. The full time duration of the graph shown might be say, maybe about 1/300 second. The marked point P/2 is the half intensity point, maybe about 1/1000 second of time duration T1 (see more description). 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, but instead it emits a continuous stream of flash pulses at a very fast rate, 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. This is still quick, so humans still perceive it as a flash, but it is a continuous light relative to the shutter duration.

The duration of this continuous FP flash (shutter curtain travel time) is in the same ballpark as the maximum sync speed. Any fully open frame also has to be opened and closed, so exposure adds the finite travel duration, open and close (exposure combines them to be one curtain's time). There is some minimum exposure when fully open, since exposure is lengthened by travel time. 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 one travel time (making Maximum Sync Speed be 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 - so they are both slightly longer than the travel time. This FP flash may still seem brief to human eyes, but it is much 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:

• FP flash is longer duration than the shutter speed, so a fast shutter speed will decimate its exposure (same as it does sun light).
• Continuous light has no motion stopping ability, the shutter is on its own.
• The output level is necessarily lower power, to be able to sustain it longer.

FP flash mode is not a new feature, it has been in most Nikon speedlight models since the SB-25 in 1992 (that user manual called it FP High-Speed Sync, and it only worked in Manual flash mode then). The idea is that this FP mode acts like the old longer-burning FP flash bulb (a longer flash duration), so that it can use any shutter speed, simply because continuous light has no sync requirement. For example (assuming ISO 200 in bright sun), we can use FP flash at any of these equivalent exposures:

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

This is NOT regular flash - These combinations are equivalent exposures, for the sun, and now, also equivalent for the FP flash. All of these are the same FP flash power level (it is a continuous light now, just like using a desk lamp). Note again, that this is NOT remotely like regular flash mode works, but this FP mode can use fast shutter speeds which can allow f/2.8 with flash in bright sun, if so desired. The wide aperture is really its only purpose. Yes, it also allows faster shutter speeds with "flash", but that is only to allow f/2.8 with the ambient light. It is NOT about stopping motion, FP flash is continuous light (cannot stop motion), and the regular speedlight flash is much faster than any shutter speed, in the same situation. And the distance range may be too short. FP flash is something you really have to want.

So what Auto FP mode does, is to allow flash fill with a fast shutter, which allows a wide aperture in bright sun. This is its purpose. If you want f/2.8 in bright sun with fill flash, this is the only way. It has no motion-stopping properties - the flash mode is no longer a speedlight. It seems worst possible choice in indoor lighting (insignificant ambient), where the speedlight mode is more powerful, and so much faster than the shutter speed. But it will allow f/2.8 with flash in bright sun (at shorter range).

Auto FP (or HSS) is not "flash", it is Continuous light

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 decimates the FP power, same as it decimates 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, 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 naturally sees half of the continuous light. And the very reason we use FP (fast shutter speed) decimates its power even more - 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 becomes slightly less than the internal flash power level: At ISO 100, SB-700 24mm zoom FP Guide Number 34. D300 internal flash Guide Number 39 (very different flash modes however). However, you can of course always use multiple FP flash units to recover some of the power (every time you double the number of flashes, you gain one stop power and exposure).

The big factor we ought to realize is that the FP range (short as it is) is the same range at any equivalent exposure (because FP is continuous light). Equivalent exposures are equivalent, for FP flash too. Regular flash is nothing like that - regular flash exposure is not affected by shutter speed, so aperture is all important. FP flash, aperture does not matter, so long as shutter speed is compensated to give equivalent continuous light exposure. The Inverse Square Law is always the same factor of course, but equivalent exposures are equivalent in FP flash mode.

HSS is definitely NOT High Speed Flash, it is merely called high speed Sync - simply because continuous light has no sync requirements. 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 - which makes no sense for continuous light which has no sync.

What is Auto about FP?

Even in Auto FP camera mode, when at actual shutter speeds not exceeding maximum shutter sync speed, everything automatically shifts back to regular flash mode, like it always was. It is called Auto FP mode because the mode depends if the actual shutter speed exceeds maximum sync speed or not. The flash unit switches to FP mode (continuous light) when the shutter speed exceeds the shutter's maximum sync speed. But anytime the camera menu is in Auto FP mode (except for SB-700 I think), then the flash LCD always displays "FP" at any shutter speed, regardless if the shutter speed is actually setting FP 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. The FP mode is actually selected when the shutter speed is actually faster than the maximum sync speed.

This mode needs our full attention. Surely Auto FP is only used in bright ambient, and then simple metering changes affecting shutter speeds (camera A mode, if near maximum sync speed) can switch this flash mode on or off - 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), but P mode understands both requirements, for ambient and flash. However P mode will go to f/22 (or f/32 on those lenses with it) before it will allow a shutter speed fast enough to switch into FP mode. That is almost as good as turning Auto FP mode Off.   :)   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 shift happening unexpectedly, unaware. FP 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 mode still works too

So I left 1/200 f/16 off this FP equivalent exposure list above, because it is not equivalent to the others in FP mode. Any shutter speed not exceeding maximum sync speed (assumed 1/200 second here) will be regular flash mode. Regular flash is nearly instantaneous (and it stops motion well), and can also achieve full power levels, and is 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. 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 at least 4x 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!" Yes, this can allow wide aperture, and shutter speed could be useful for motion in bright daylight (if you have the distance range).

But a SB-600 at 1/32 power (for close range) flash duration is 1/20,000 second! (spec chart in rear of flash manuals, SB-600 page 88). For example (page 35), ISO 400, zoom 35mm, 1/32 power, is GN 17.4x2 = 35, for f/8 at 4.3 feet, or f/4.3 at 8 feet - this at 1/20,000 second if 1/32 power. Regular flash mode and maximum shutter sync speed of course. This speed can tame camera shake for macro work, but it cannot help any focus shift swaying front to back.

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.

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Auto FP is NOT Fast Flash (anything but)

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 decimate its power, but the f/3 aperture compensates for that aspect.

If 400 revolutions per second, 1/41600 second flash computes 3.5 degrees of rotation. And 1/8000 second 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. The inked lines rotated while the shutter slit was moving down the FP frame, which distorts where they are shown to be (I believe the shutter travels downwards, which is inverted in the image, so it travels upwards in the picture).

Make no mistake, FP mode flash is NOT fast flash, it is the furthest thing from it. 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.

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Electronic shutters

Someone will of course say "but the old D40/D50/D70 cameras had an electronic shutter, and sync was 1/500 second, and for manual flash not on hot shoe, these can sync at ANY fast shutter speed". Which is correct, and there can be advantage. Faster shutter speed with regular flash is the obvious desirable answer. It just is not going to happen on a focal plane shutter.

And this electronic shutter is not perfect anyway. The issue is that these cameras have a slow mechanical shutter, maybe around 1/100 second, which is used for slower speeds. It blocks light off of the CCD sensor, so of course, it has to open for faster speeds, but which then are timed by the electrical shutter. The CCD chip is simply enabled, and then disabled, to simulate a shutter (this disable after each frame is a necessary part of CCD operation anyway, to prevent blur when shifting the image out of it). This means at say 1/4000 second electrical shutter, the mechanical shutter is still open for 1/100 second (a very long time), allowing the light to still hit the disabled CCD sensor. The light (intense overexposure, so to speak) is on the disabled sensor, and can cause blooming in the CCD image. About all compact cameras still do this (free in the CCD is much cheaper than a fancy mechanical focal plane shutter), but better DSLR seem to have abandoned it now. Not all CCD DSLR models chose to use a chip shutter - some CCD models (D40X, D60, D80) instead add the better focal plane shutter (with 1/200 second sync). And CMOS sensors don't implement the electronic shutter anyway, it's been too slow except for video (new Nikon J1 mirrorless camera tries, with 1/60 second flash sync. The V1 uses focal plane shutter, 1/250 second sync).

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Regarding FP flash reduced power level

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.

FP flash has two disadvantages. It is continuous light at much lower power level, and even then it is decimated more by shutter speed. And it 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.

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.

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Some flash Power 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. Unfortunately some minor clouds, some minor variance, but I tried. 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 much.

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. Fill flash, notice the top right corner.
The flash LCD range says 8.8 feet (SB-800 Guide Number at 24mm is 98 x 1.414 = 138 ISO 200, divided by f/16 is 8.7 feet range). 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 (41% farther is -1EV), and we still have shadow, but a lighter shadow, an appropriate fill level.

Above: Regular TTL flash -1 EV flash compensation, 1/200 second f/16. The LCD range says 12 feet (range for -1EV fill).

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. It is just not much range. But fill does not need as much power as a sole light source would need, as fill is expected to be down about a stop (real fill range is 40% more range... or we could instead use -1EV flash compensation). And note that the regular flash mode LCD above reported only 8.7 feet range, and was still usable as fill at 12 feet (but 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 (range for -1EV 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 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.

Above: Again, no flash. 1/250 second f/16. Notice the bricks in upper right. Fill level was significant at 12 feet in bright sun.

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