Flash

Noon on a pale beach, a friend with the sun behind her, and the Nikkor Z 85mm f/1.8 S wide open so the dunes melt into blur. Say the meter asks for 1/4000 s at f/1.8 and ISO 100. Her face sits in her own shadow and a flash would lift it, but at its default setting the Z6III will not pair a flash with any speed faster than 1/200 s (pp. 417, 625, 690). The gap is $\log_2(4000/200) \approx 4.3$ stops. Stopping down to f/8 would close it, since $(8/1.8)^2 \approx 19.8$, and would also pull the dunes back into focus, the one thing the picture did not want.

Nikon's specifications list no built-in flash, only an accessory shoe built to the ISO 518 standard, carrying sync and data contacts, and the Reference Guide treats every flash as an optional unit on that shoe or a remote unit under wireless control (pp. 415, 426, 1003). Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.How Flash Light Behaves

Distance and the Inverse-Square Law

A small flash head throws its light out in a widening cone. Each doubling of distance spreads it over four times the area, so each patch of the subject receives a quarter as much. The illuminance $E$, the light landing on each unit of area, follows the inverse-square law, with $d$ the distance from the light:

$$E \propto \frac{1}{d^2}$$

One EV (exposure value), also called a stop, is a doubling or halving of light, so moving from distance $d_1$ to $d_2$ changes the flash by

$$\Delta \mathrm{EV} = 2\log_2 \frac{d_2}{d_1}$$

With a face 1.5 m from the flash and a wall at 3 m, the wall receives $2\log_2 2 = 2$ EV less flash. Move the light to 0.75 m from the face, which leaves the wall 2.25 m away, and the gap grows to $2\log_2 3 \approx 3.2$ EV. A closer light darkens the background relative to the subject, which makes distance the cheapest backdrop control there is. The law is exact only for a point source and a softbox used close up follows it loosely, but the direction holds.

Guide Number

A flash's guide number (GN) folds its full power into one figure: the subject distance in metres multiplied by the f-number $N$ that exposes correctly at ISO 100, for a stated zoom setting of the head.

$$\mathrm{GN} = d \cdot N$$

The product is constant because halving the distance delivers four times the light while doubling $N$ admits a quarter as much, since the light through the lens varies as $1/N^2$. An imaginary flash with GN 40 reaches 10 m at f/4 and 5 m at f/8. Raising ISO from 100 to 400 quadruples the sensitivity, which the inverse-square law turns into twice the reach, so the guide number grows with the square root of the ISO $S$:

$$\mathrm{GN}_S = \mathrm{GN}_{100}\sqrt{\frac{S}{100}} = 40\sqrt{\frac{400}{100}} = 80$$

The flash is imaginary because Godox's V100 page, read on 2026-10-01, gives no guide number. It quotes stored energy instead, 100 Ws at full output, where a watt-second (Ws) measures the energy the flash stores, and output from 1/1 to 1/256, a span of $\log_2 256 = 8$ stops.

Two Exposures in One Frame

A flash picture is two exposures laid on top of each other. Ambient light from the sun or the room arrives for as long as the shutter is open. The flash arrives in one short burst, which Godox rates for the V100 at 1/300 s to 1/20000 s, so provided the whole burst lands while the sensor is uncovered, shutter speed has no say in how much flash is recorded. At full power, though, a 1/300 s burst may outlast the sensor's fully open moment at 1/200 s, my inference. Flash power and distance therefore change only the flash part, shutter speed only the ambient part, and aperture and ISO both by the same amount. At f/2.8 and ISO 100, slowing from 1/200 s to 1/50 s brightens a dim background by $\log_2 4 = 2$ EV and leaves the flash on a nearby face where it was. Stopping down to f/5.6 instead darkens both by 2 EV, and the flash then needs four times the output.

02.The Sync Speed

Why the Shutter Sets a Limit

The Z6III's mechanical shutter is a pair of curtains travelling vertically just in front of the sensor (p. 1000). The front curtain uncovers the rows and the rear curtain follows to cover them, so every row is exposed for the same time but from a different moment, as Shutter Types, Rolling Shutter and Flicker works through. Flash needs an instant when every row is uncovered at once. At 1/200 s and slower there is one, and the flash fires then. Faster, the rear curtain sets off before the front one finishes, the sensor sees the scene through a moving slit, and a single burst would light only the rows inside it. Nikon puts flash sync at 1/200 s and slower (p. 1000).

Flash sync speed offers 1/200 s (Auto FP) and fixed speeds from 1/200 s down to 1/60 s (p. 690), with 1/200 s the default (p. 625). With a fixed setting, a faster speed chosen in shutter-priority auto (S) or manual (M) is pulled back to the sync speed (p. 690). With Electronic shutter chosen in Custom Setting d6, e1 cannot be set at all (p. 668).

Auto FP High-Speed Sync

1/200 s (Auto FP) enables Auto FP high-speed sync, Nikon's name for flash above the sync speed, with units that support it. The Reference Guide never spells out the letters FP. In S and M you may then choose up to 1/8000 s, or 1/2000 s with d6 at Electronic front-curtain shutter (EFCS, where each row's exposure starts electronically and only the rear curtain moves). In programmed auto (P) and aperture-priority auto (A), it engages whenever the speed the camera picks is faster than 1/200 s (p. 690). The 1/2000 s cap applies only when EFCS is selected, so the opening scene's 1/4000 s works at d6's default, Auto, which First-Day Setup keeps.

The Reference Guide does not describe the mechanism. The usual account, which the right half of Figure 1 follows, is that the flash fires a rapid train of small pulses while the slit crosses, so it acts like a steady lamp. Nikon's caution that horizontal lines can appear at some speeds, eased by a slower speed or a change of output (p. 691), fits that picture.

The cost in range follows from the same picture, and this part is my reasoning rather than anything Nikon or Godox states. Each row collects flash only during its own exposure, as it collects sunlight, so each stop of faster shutter costs a stop of flash. From 1/500 s to 1/4000 s is $\log_2 8 = 3$ EV, which by the inverse-square law shrinks the reach at fixed output and aperture by $2^{3/2} \approx 2.8$. Above the sync speed the shutter dims flash and sun together, so the balance between them comes from flash output and distance. Nikon's own tip for the feature is the opening scene: keep the widest aperture with a subject backlit by bright sun (p. 691).

Front-Curtain, Rear-Curtain and Slow Sync

Flash mode decides when the flash fires and how far the shutter may slow around it (pp. 421-422). The default, Fill flash (p. 447), fires as soon as the front curtain opens, which is called front-curtain sync, and in P and A holds the shutter between 1/200 s, or faster with Auto FP, and 1/60 s (p. 421). Red-eye reduction lights a lamp or fires the flash first, and Nikon advises against it with moving subjects (p. 421). Slow sync and Slow sync + red-eye, in P and A only, let the shutter slow, as far as 30 s (p. 417), to record a dark background, with a tripod recommended. Flash off stops the flash (p. 422).

Rear-curtain sync, in P, S, A and M, fires just before the shutter closes (p. 422). Picture a cyclist with a rear lamp crossing the frame during a 1 s exposure. Front-curtain sync freezes the rider at the start, and the lamp then streaks ahead of the frozen figure. Rear-curtain sync draws the streak first and freezes the rider at the end, so the trail follows behind. In P and A it becomes slow rear-curtain sync, and Nikon warns that it may not synchronise with studio flash systems (pp. 422, 692).

Flash shutter speed sets the slowest speed P and A will use with flash, 1/60 s by default (pp. 625, 692). It does not bind S or M, which can reach 30 s, nor the slow-sync settings (p. 692). With a flash, an Auto ISO Minimum shutter speed outside the e1 to e2 range gives way to e2 (p. 162), as ISO and Auto ISO notes.

Shutters and Modes That Rule Flash Out

With Electronic shutter in d6, flash syncs only at 1/60 s and slower, without Auto FP (pp. 691, 1000). Flash cannot be combined with Silent mode, and Nikon says to set it to OFF before using any flash, mounted or remote (pp. 415, 966). Silent mode forces the electronic shutter, and a mounted flash will not fire in it (p. 846). Flash is also ruled out with video recording, high-speed frame capture + or HDR (high dynamic range) overlay (pp. 107, 157, 966), with Interval priority at ON (p. 523), or with a focus shift interval of 00 (p. 550).

03.How i-TTL Sets the Power

TTL means through the lens: the camera judges the flash by light that has come through the lens. i-TTL is Nikon's version, part of the Nikon Creative Lighting System (CLS), its family of flash features and compatible units, and it needs a CLS flash set to TTL (p. 417). Rather than measure the main burst, the unit fires a series of weak monitor pre-flashes after a full press of the shutter-release button and just before the main flash, and the camera reads their return to set the output (p. 417). With matrix, centre-weighted or highlight-weighted metering that is i-TTL balanced fill-flash, which balances subject against background. Spot metering switches to standard i-TTL fill-flash, which ignores the background and which Nikon recommends when the subject should dominate or exposure compensation is in use (pp. 417-418, 1003), as Metering noted.

Nikon gives i-TTL a range of ISO 64 to 12800 equivalent (p. 965). A blinking flash-ready indicator after a shot means the flash fired at full output and the picture may be dark (p. 924). Nikon also warns that diffusion panels other than the flash's own can upset i-TTL exposure (p. 965), and says nothing either way about softboxes.

Flash Compensation and Exposure Compensation

Flash compensation changes the flash output alone, from −3 to +1 EV with 0.0 the default, in steps set by Custom Setting b2 (pp. 423, 447, 653). An icon shows while it is not zero and the value survives power-off (p. 423). In i-TTL and auto aperture (qA), compensation set on a Nikon flash or in Flash control adds to it (p. 967).

Exposure comp. for flash decides what ordinary exposure compensation does when a flash fires. Entire frame, the default (p. 625), moves flash and ambient together (pp. 152, 693). Background only moves the background alone (p. 693). On a fill-flash portrait against a bright sky, −1 EV under Entire frame darkens face and sky by a stop. Under Background only the sky darkens and the face keeps its flash, which is usually the point.

FV (flash value) lock fixes the output after one pre-flash so that you can recompose (pp. 424-425). It needs a CLS flash in TTL, or in qA or non-TTL auto with monitor pre-flash, is not available in AUTO, and needs a control given FV lock under f2 (pp. 424, 706). This guide's button map assigns none.

Auto ISO With Flash

Auto flash ISO sensitivity control sets what Auto ISO aims at with flash (p. 694). Subject and background, the default (p. 625), weighs both, and Subject only exposes for the main subject. Nikon does not spell out the trade-off. My reading is that the first lets ISO rise to hold a dark backdrop and the second lets the backdrop fall away. The ceiling is Maximum sensitivity with flash, default Same as without flash (pp. 446, 467). With d10 at Show effects of settings > Always, TTL and Auto ISO together can make the display misjudge the subject's brightness (p. 673), as Viewfinder and Monitor Displays explains.

04.Nikon's Flash Menus

Flash control holds Nikon's flash settings (pp. 488-490). Flash control mode, default TTL (p. 447), also offers Auto external flash, where a sensor on the flash reads the reflected light in its qA (auto aperture) or A (non-TTL auto) form, Distance-priority manual, which sets output from a subject distance, Manual, and Repeating flash, several bursts in one exposure (pp. 419-420). Only four Nikon units, the SB-300, SB-400, SB-500 and SB-5000, accept these settings from the camera. Every other flash is set on its own controls (pp. 419, 488).

Wireless flash options chooses between the two forms of Nikon's remote-flash system, Advanced Wireless Lighting (AWL), or turns it Off, the default. It appears only when the camera carries an SB-500 or SB-5000, or has a WR-R10 or WR-R11b controller attached (pp. 447, 489). Optical AWL commands remote units by light from a master flash on the shoe, and Radio AWL by radio from the WR controller (pp. 426-427, 438). Remote flash control, default Group flash, and Radio remote flash info serve those systems (pp. 447, 489-490).

Wireless remote (WR) options serves the WR-R11b and WR-R10 (p. 851). LED lamp, the controller's light-emitting diode (LED) indicators, defaults to ON. Link mode defaults to Pairing, where the camera talks only to devices paired with it, against PIN, where all devices sharing a four-digit PIN communicate and the first camera to connect takes sole control (pp. 807, 851-852). A WR-R10 needs firmware 3.0 or later (p. 851).

Modeling flash, default ON (p. 625), makes a CLS flash emit modelling light when a control given the Preview role under f2 is pressed (p. 695), and Nikon advises judging the shadows by eye (p. 968). This guide's button map assigns no Preview control.

Flash burst priority governs flash in the continuous release modes (p. 698). Prioritize precise flash control, the default (p. 625), pre-flashes before every frame and the frame rate may drop. Prioritize frame advance rate pre-flashes once, holds that output for the burst and slows less. Continuous H (extended) always favours the rate (p. 698), as Release Modes, Frame Rates and the Buffer notes.

05.Off the Camera With Godox

My flash is a Godox V100N, a round-head unit in Godox's Nikon version, fired through a Godox X3 Pro-N radio trigger on the hot shoe, as A Tour of the Body mentions. For portraits it goes in an 80 cm octagon softbox with a grid on a light stand. Neither is a Nikon product, so what each does comes from Godox, and what each does on the Z6III is my inference wherever Nikon is silent.

Godox rates the V100N for i-TTL and manual flash, high-speed sync to 1/8000 s, first- and second-curtain sync, and its 2.4G Wireless X system on 32 channels over about 100 m. It rates the X3 Pro-N for TTL, manual and multi flash, high-speed sync and both curtain modes over 2.4 GHz radio, with 16 groups and flash compensation from −3 to +3 EV on the trigger. For the N version it adds one note: second-curtain sync is set on the camera.

The shoe carries sync and data contacts (p. 1003). The sync contact only says fire now. TTL needs the data contacts, and Nikon promises i-TTL only for CLS units (p. 417), a term Godox's pages never use. My inference is that the trigger answers the camera on the data contacts as a compatible flash would, relays the result over radio, and that e1 is what lets the chain go above 1/200 s. Neither company documents that chain. Nikon's only third-party warning concerns units that put more than 250 V on the X contacts or short the shoe (p. 417), which Godox's page does not address.

The rule I take from it: the camera decides when the flash fires and, in TTL, how bright the picture should be, while the trigger sets the flash's mode and output. For colour, the Flash white balance preset is 5400 K (p. 165), as White Balance covers.

06.Where I Use Flash and Where I Do Not

What follows is my judgement, not Nikon's advice.

Portraits come first: the V100N in the softbox, close to the face and to one side, with the 85mm f/1.8 S in A as Exposure Modes recommends. With the box near the face, a background a few metres back falls off by several stops, and the grid keeps light off it. I would start in TTL to get close quickly, then switch the trigger to manual, because a manual output stays put when the framing changes while TTL re-meters every frame.

Fill in hard midday sun is the second use, and the reason for 1/200 s (Auto FP). In A at f/1.8 the camera can run past 1/200 s while the flash, at negative compensation, lifts the shadows. The lost reach means the light goes close.

I do not use flash at stage performances. A burst in a dancer's eyes mid-turn or mid-lift is a safety risk, it pulls the audience out of the show, and venues forbid it. Stage work runs in silent mode anyway, which Nikon does not allow with flash (pp. 415, 966).

I do not use flash on wildlife either. A burst can startle an animal, and at the distances the Nikkor Z 180-600mm f/5.6-6.3 VR works, a hand flash has little to give. The imaginary GN 40 flash at f/6.3 and ISO 100 reaches about 6.3 m, so a bird at 30 m sits $2\log_2(30/6.3) \approx 4.5$ stops beyond it.

07.Recommended Settings

08.Beginner Mistakes

Leaving silent mode on. Nikon rules out flash in Silent mode (pp. 415, 966). Turn it off with the Illuminator button, in this guide's map, before any flash work.

Expecting high-speed sync at the default. At 1/200 s the camera holds the shutter at the sync speed (p. 690). Choose 1/200 s (Auto FP) in e1.

Forgetting flash compensation. It survives power-off (p. 423). Check for the icon, and set compensation in one place, on the trigger.

Setting a Godox flash from the camera's Flash control menu. Those controls reach only four Nikon units (p. 488). Use the X3 Pro-N.

Using the electronic shutter with flash. Sync drops to 1/60 s and Auto FP disappears (p. 1000). Return d6 to Auto, since EFCS would cap Auto FP at 1/2000 s (p. 690).

09.Related Chapters

• Shutter Types, Rolling Shutter and Flicker: how the curtains move and why they cap the sync speed.
• Exposure Modes: P, S, A and M, exposure compensation and flash bracketing.
• Metering: the metering modes, including spot metering's switch to standard i-TTL.
• ISO and Auto ISO: Maximum sensitivity with flash and the minimum shutter speed.
• White Balance: the Flash preset and Auto under flash.
• Silent Shooting, Timers and Time-Lapse: silent mode and why it rules out flash.
• Custom Controls: the button map, including the Illuminator button.
• Long Exposure and Night: slow shutter speeds and long exposures in general.