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.

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, typically around the 1/200 second ballpark (can vary with camera model.) This is not a big issue for flash indoors (the speedlight is faster than the shutter), but it is more issue with fill flash in bright sun (explained here.) To aid that purpose, many 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 - 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.

The FP flash concept is a rather special situation, frankly, often not what you want. 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 actual shutter speed is faster than maximum shutter sync speed. Camera A or P mode sets shutter speed automatically, and normally will not provide a fast shutter speed in dim light indoors (light is not bright enough to meter a fast shutter speed), so camera M mode will be necessary indoors. Frankly, that would be silly, Auto FP (HSS) mode is never what you want indoors (counterproductive), because the speedlight is much faster than the shutter speed. 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 you really ought to know...

Nikon camera models offering Auto FP include D80/D90, D200/D300, D700, D7000, and D3 (often menu around E1.) The internal flash cannot do Auto FP mode, nor can the SB-400 flash. But the SB-600, SB-700, SB-800, SB-900, SB-910 and SB-R200 can, on either the hot-shoe, or as a remote flash via Commander. Bright sun should help prevent blinking, but Commander mode may still need FV Lock to prevent subject blinking.

Note: The camera internal flash cannot do FP flash. It can 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 (builtin 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.

What is a focal plane shutter?

Focal plane shutters have been used in the better 35mm cameras since the first Leica in 1925, because they provide superior timing. This system is great for interchangeable lenses, because it only needs one good shutter in the body, instead of a shutter in every lens. And this is a better shutter, but FP shutters do have the one limitation of maximum shutter sync speed with flash.

A focal plane shutter is two moving shutter curtains in front of the sensor. At slower shutter speeds, one curtain opens (to expose the sensor to light from lens), and after a timed duration, the second curtain closes. That makes the full frame open to the lens for the duration, which allows the instantaneous flash to work. Flash is fast, and it can occur anywhere within this duration, but normal mode is early front curtain sync, and we have later rear curtain sync (Part 5.)

But at faster shutter speeds, the second curtain starts closing before the first is fully open, tracking together to create a narrow timed slit between the two curtains, the slit moving across the frame (down the frame today, shorter travel is faster.) The overall shutter travel movement takes longer (always a constant time, approximately near maximum sync speed time, maybe 1/3 second.) The point is, a slower mechanical mechanism moves both curtains, making and moving a virtual narrow slit of opening across the frame, to provide precise fast exposures under that slit. The geared motor travel speed of the two curtains is always at one fixed speed, which can be made very accurate. Today, the timing between the curtains is a quartz clock. The exposures of different parts of the frame under this moving slit may occur at slightly different times, but the actual exposure under this moving open slit can be very fast, and very accurate.

Wikipedia describes focal plane shutters in more detail, and an interesting classic picture using a focal plane slit is this 1912 picture by Henri Lartique. The shutter slit is moving down the film, and the spectators are leaning one way because the 4x5 camera was being panned the other way following the race car, but the car was going faster yet. This classic picture is responsible for us imagining speeding wheels as being slanted ovals, at least in cartoons. Shutters are faster today, but fast shutter speeds with flash can still be an issue.

The downside for flash is at these faster shutter speeds, the shutter opening (of light to the sensor) is only under this narrow slit. An instantaneous flash then only illuminates what is under that narrow slit at that instant, and the rest of the frame is not exposed to the flash. This is generally a problem, and the camera normally will not let us set shutter speeds faster than maximum sync speed.

What is Maximum Shutter Sync Speed?

For focal plane shutters, the fastest shutter speed at which the full frame is all fully open at any one instant (to allow flash), is called the Maximum Flash Sync Speed (I often say Maximum Shutter Sync Speed, meaning the same thing - maximum shutter speed for flash sync - about syncing flash with the open shutter.) Proper flash sync is not possible if the shutter speed is faster than this limit (some of the frame would be covered by the shutter, and so unexposed, causing a dark band in picture.) Until up into the late 1980s, FP maximum sync speed was 1/60 or 1/80 second. Today, it is 1/200 and 1/250 second, which is pretty good. Any slower shutter speed can be used too, causing no issue regarding sync.

No big deal indoors, but this maximum sync speed becomes a common problem when using fill flash in bright sun.

The Problem with Fill Flash in Sunlight

The Sunny 16 Rule says at ISO 200, exposure in bright sunlight is 1/200 second at f/16. Bright sun does not vary, so this is the norm we expect. And we pretty much have to correctly expose the daylight scene - we cannot ignore the sun like we can ignore dim indoor illumination.

With flash in sunlight, we cannot use faster equivalent exposures, like 1/400 second at f/11, or 1/800 at f/8 - because the focal plane shutter's maximum sync speed is 1/200 or 1/250. That means, to use flash in bright sun, ISO 200 WILL BE around 1/200 second at f/16 (Sunny 16). The f/16 requires a lot of flash power, but otherwise this works fine, unless you crave to use f/2.8 out there. Camera P mode knows all about this, and has ability to set both shutter speed AND aperture, and so it is a good choice for fill flash in bright sun. But if you use camera A mode, and set f/4 out there without thinking, the camera will just fuss warning HI at you, until you set near f/16 so it can work.

Wishful thinking, but our dream is that if somehow we could increase shutter speed, we could open the aperture, for reduced depth of field, or to an equivalent exposure for daylight which lets the regular flash mode work at lower power level, without affecting either exposure. Or, a faster shutter speed could help the flash to "overpower" the sun, reducing the sun without affecting the flash - if we could, but we cannot (in these bright sun cases..) We are up against the maximum shutter sync speed wall. FWIW, using lower ISO, or using a Neutral Density filter, won't help much. Yes, these can allow a wider aperture in this case, but that's all it does (maximum shutter sync speed is still enforced.) But no, these affect the flash too, same as they affect the sun, so these do not change the balance between flash power and sun. We can only use more flash power to affect the balance, and then exposing the higher flash power might end up at f/32, but which does decrease the sun that needs f/16. Flash in bright sun is a special case.

Not the same thing, but Auto FP mode does offer one way to increase shutter speed with flash.

What is Auto FP flash mode?

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 slowly, to provide light for a relatively long time, so that no matter where this narrow shutter slit was in its travel across the frame, 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. 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 flashes, at a very high repeating rate, so that they run together and simulate 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. Humans still perceive just a flash, but the continuous light is significantly longer now, it lasts for the full shutter travel time (perhaps about 1/3 second, even with an extremely fast shutter speed - these are ballpark approximations, not precise.) The camera FP mode triggers this flash slightly before the shutter opens, instead of after the shutter opens, so the shutter sees continuous.

So today, this Auto FP mode is sometimes provided. 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 flashes, at a very high repeating rate, so that they run together and simulate 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. Humans still perceive just a flash, but the continuous light is significantly longer now, it lasts for the full shutter travel time (perhaps about 1/3 second, needed with any extremely fast shutter speed - these are ballpark approximations, not precise.) The camera FP mode triggers this flash slightly before the shutter opens, instead of after the shutter opens, so the shutter sees continuous.

This FP flash mode is radically different than "flash". FP mode can not change the shutter action, nor override the maximum sync speed, but instead it changes the physical flash unit - which is no longer a speedlight, but instead it emits a continuous stream of flash pulses which mimic continuous light, for the duration of the shutter slit travel time. More like a desk lamp now. This is a huge difference.

This FP mode is like the old longer-burning FP flash bulb, and we can use any shutter speed, simply because continuous light has no sync requirement. For example (assuming ISO 200 in bright sun), we can use 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 major purpose, but it also allows faster shutter speeds with flash - however the distance range may be too short (action is usually not very close.)

Auto FP 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. Auto FP is continuous light, longer then the shutter duration, truncated by shutter duration. 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".)

However, the FP flash power is substantially reduced (to be able to be on continuously), to less than 1/4 power maximum, and the maximum distance range is substantially reduced. 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.)

This paragraph is probably just a confusion factor, but how much is the FP range reduced? Regular flash, we know 1/4 power is 1/2 distance range (inverse square law.) Regular flash, opening aperture wider increases effective flash power and range - since shutter speed is not a factor, so less power is required. But FP mode begins at less than 1/4 power, and FP mode is continuous light, which is affected by shutter speed too - that is, if we compensate aperture wider, we must compensate shutter speed faster too (equivalent exposure), so there is no power advantage of aperture. We give up very much by using FP flash mode. The only one advantage of FP is ability to use flash (what we may still erroneously call flash) for fill in bright sunlight, at wide aperture and equivalent exposure high shutter speed, if the distance range is short enough.

The 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.) Regular flash is nothing like that - regular flash exposure is not affected by shutter speed, so aperture is all important.

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 - it 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. Simple metering changes affecting shutter speeds (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, and we should 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), 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 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. 1/200 second shutter and slower will be regular flash mode. Regular flash is nearly instantaneous (and it stops motion well), and can also achieve full power levels.

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, in all of 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 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 this. Speedlights are the basis of high speed flash photography. We do have better tools available than some may realize.

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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 cannot implement the electronic shutter anyway (new Nikon J1 mirrorless camera tries, with 1/60 second flash sync. 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 a fast shutter and a wide aperture in bright sun. It 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 Speed examples

A hand grinding tool, the disc is one inch diameter. The Black&Decker manual claims 24,000 RPM (400 revolutions/second.)

D300, (manual mode), ISO 200, with SB-800 (manual mode), on camera hot shoe at about 33 inches.

Regular speedlight mode, at 1/128 power. f/3 1/250 second. First stopped, and then turning 24,000 RPM.	FP flash mode, at 1/2 power. f/3 1/8000 second. First stopped, and then 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 for the flash, but the flash is only on a very short time.	The FP frame does not even stop the center screw head. Weird distortion effects from the focal plane shutter (ink lines are not at 90 degrees) because the FP frame has a 1/8000 second slit moving up the frame during perhaps 1/3 second travel time.

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 in perhaps 1/3 second.

It seems a very major point that using speedlight flash is how we achieve high speed photography, like say stopping water drop splashes. See maybe this.

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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. 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 ligher 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 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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