Multiple Exposure, HDR Overlay and Pixel Shift

Thirty-two raw files of a carved wooden door, each 6,048 by 4,032 pixels, can become one picture 12,096 by 8,064 pixels. Two frames of a dancer, arms raised in one and mid-turn in the other, can become a single frame in which she appears twice. One press at dusk, on a tripod, can give a JPEG that tries to hold both lit cloud and shadowed rock. All three come from the same move: several exposures, combined by arithmetic.

01.A Picture Is a Grid of Numbers

Start with one pixel. In a JPEG, each pixel stores three numbers, for red, green and blue, and each runs from 0 to 255 because JPEG keeps 8 bits per colour and $2^8 = 256$, as RAW, JPEG, HEIF and Tone Mode explains. A Large FX picture is a grid of $6{,}048 \times 4{,}032 = 24{,}385{,}536$ such pixels.

Take two frames from a tripod without touching the camera. The pixel at column 3,000, row 2,000 sees the same patch of scene in both. Combining the frames means applying one rule at all 24 million positions: add the readings, average them, keep the larger or keep the smaller. That is a multiple exposure. HDR (high dynamic range) overlay is a version in which the camera itself decides the blend. Pixel shift moves the sensor a tiny distance between frames, so each samples a slightly different patch, and software interleaves the readings into a finer grid.

The worked examples use one invented brightness number per pixel on that 0 to 255 scale. Nikon publishes each rule but not the camera's internal arithmetic, so read the values as a model of the rule.

02.Multiple Exposure

Multiple exposure joins two to ten exposures into one photograph (p. 511). Each is taken as a NEF, Nikon's raw format.

The Options

Multiple exposure mode offers On (series), which keeps making multiple exposures until you choose Off, On (single photo), which makes one and returns to normal shooting, and Off (p. 511). Number of shots runs from two to ten. Overlay mode sets the arithmetic. Save individual pictures (RAW) at ON keeps every exposure as its own NEF besides the combination, and at OFF keeps only the combination (p. 512). Overlay shooting at ON lays earlier exposures over the live view as a guide. Select first exposure (RAW) uses a NEF already on the card as exposure one, and shooting then starts at exposure two (pp. 512, 514). The defaults are Off, two shots, Average, ON, ON and no first exposure (p. 447).

The Four Overlay Modes on One Pixel

Call the readings of one pixel across $N$ exposures $v_1, v_2, \dots, v_N$. Nikon's four modes (p. 511) are four formulas.

$$\text{Add} = v_1 + \dots + v_N \qquad \text{Average} = \frac{1}{N}\left(v_1 + \dots + v_N\right)$$ $$\text{Lighten} = \max(v_1, \dots, v_N) \qquad \text{Darken} = \min(v_1, \dots, v_N)$$

Add stacks the exposures without changing the gain, the multiplier applied to each reading. Average sets each exposure's gain to $1/N$, so two exposures count for $\tfrac{1}{2}$ each and three for $\tfrac{1}{3}$. Lighten keeps the brightest reading at each pixel and Darken the darkest.

Take three pixels from a stage. A sequinned sleeve reads 180 in exposure one where the black backdrop reads 40 in exposure two. A dancer crosses a second spot in one of three exposures, reading 200 there and 30 in the others. A third spot reads backdrop, 30, three times.

Add keeps all the light: $200 + 30 + 30 = 260$ overruns the scale and clips to white, and the backdrop triples to 90. If the camera adds linear readings, numbers proportional to the light as raw data are, which Nikon does not say, $N$ equal exposures carry $N$ times the light, or $\log_2 N$ stops since a stop is a doubling: one for two shots, $\log_2 3 \approx 1.58$ for three. On that inference, not Nikon's advice, I would darken each exposure by about that much when using Add on anything but black.

Average can never exceed its brightest reading, so it creates no clipping, but anything present in only some exposures fades: the dancer becomes $\tfrac{1}{3}(200 + 30 + 30) \approx 86.7$, a ghost. Lighten keeps her at 200 over an untouched backdrop, which suits bright subjects on black. Darken drops her to 30, so it suits dark shapes such as bare branches against a pale sky.

Shooting, Checking and Ending

With the mode on, an icon appears in the shooting display (p. 512), and the camera combines the frames once the set number is taken (p. 514). Mid-sequence, the playback button shows the shots so far, and i on the latest offers View progress, a preview of the blend, Retake last exposure, Save and exit, which builds the picture from the frames taken, and Discard and exit, which still keeps the NEFs if saving is ON (p. 515).

Choosing Off early also builds the picture, and Average then divides by the number actually taken (p. 517), so stopping at two of a planned three weights each by $\tfrac{1}{2}$. A sequence also ends when the standby timer runs out after the first exposure (p. 517), or after about 40 seconds idle in playback or 90 in the menus, which longer c3 Power off delay times for Playback or Menus extend (p. 516).

A continuous release mode takes all the exposures in one burst, and with the self-timer, c2 Self-timer > Interval between shots spaces them (p. 516). The recorded settings are those of exposure one, changing settings may end the sequence, noise can appear as specks, fog or lines, the card must not be removed and cannot be formatted, and some menu items are greyed out (p. 516). The photo shooting menu cannot be reset (p. 450) and voice memos are blocked (p. 298) meanwhile.

What Gets Saved

The combined picture is always a JPEG, whatever Image quality says (p. 514). With Save individual pictures (RAW) at ON the NEFs are kept too (p. 512). Under RAW primary - JPEG secondary, covered in Card Slots and File Handling, those NEFs go to both cards with the JPEG (p. 459). These pages give no size or compression grade for the composite JPEG.

Restrictions

Nikon names the features multiple exposure cannot share the camera with (pp. 157, 516). Two are whole modes, the auto shooting mode and video recording. The rest are the other multi-frame tools: auto capture, focus shift, pixel shift, HDR overlay, time-lapse movies, the interval timer, bracketing and HSFC+ (high-speed frame capture +, the high-rate JPEG burst modes). Multiple exposure is also unavailable under the HLG (hybrid log-gamma) tone mode (p. 462), Skin softening does not work with it (p. 480), white balance bracketing cannot be used (p. 916), and a U position cannot store it (p. 143). The list names no limit on shutter type, image area, flash or vibration reduction (VR).

03.HDR Overlay

Dynamic Range in Stops

Stand on a tripod at the edge of a gorge in late afternoon. Suppose the sunlit cloud sends 4,000 times as much light as the shadowed rock you want detail in. In stops, the scene spans $\log_2 4{,}000$, and since $2^{12} = 4{,}096$ that is just under 12. The Glossary calls the same unit EV, exposure value.

A single exposure has its own span, from the level where a photosite, one light-collecting site on the sensor, fills and records white down to where the signal drowns in noise. The stops between are the frame's dynamic range. Take an invented 10 stops, not a Z6III measurement: the scene is two stops wider, so either the cloud clips or the rock sinks into noise.

Now take two exposures two stops apart. The darker keeps the cloud below clipping, and the brighter gathers $2^2 = 4$ times the light and lifts the rock clear of the noise. Each spans 10 stops, offset by 2, so together they cover

$$10 + 2 = 12 \text{ stops.}$$

In general, two frames $d$ stops apart, each spanning $R$ stops, cover $R + d$ as long as $d$ is no larger than $R$, so the spans overlap. The merge takes each area from whichever frame recorded it cleanly.

The Settings

HDR overlay merges two shots at different exposures to keep detail from highlights to shadows (p. 518). HDR mode offers On (series), On (single photo) and Off, as for multiple exposure. HDR strength at Auto suits the strength to the scene. Save individual pictures (RAW) at ON keeps both frames as NEFs. The defaults are Off and Auto (p. 447), with saving OFF (p. 448).

Shooting and Saving

Nikon recommends matrix metering for HDR (p. 519). A full press takes two exposures, "Busy" flashes in the control panel while they are combined, and nothing can be shot until that ends (p. 520). The result is a JPEG whatever Image quality says (p. 520), and under RAW primary - JPEG secondary any saved NEFs go to both cards (p. 459).

That JPEG has 256 levels per colour, so 12 stops must be squeezed. HDR software usually does this by local tone mapping, pulling bright areas down and lifting dark ones region by region, and at a sharp border the adjustment spills across, leaving a dark rim by a bright object or a glow by a dark one. Nikon does not describe its method, but it warns of those shadows and halos, of uneven shading on some subjects, and that the effect may barely show in some scenes (p. 521).

Movement Between Frames

The two exposures are taken one after the other, so anything moving is in two places. Where a swaying branch crossed, one frame reads branch at 50 and the other sky at 220, and whatever the merge does, the edge doubles or ghosts. Nikon warns that camera or subject movement may spoil the result, recommends a tripod, and notes that the picture's edges are cropped (p. 521).

Cautions and Restrictions

With spot or centre-weighted metering, Auto strength acts as Normal (p. 521). Nikon suggests Center-weighted metering with an ND (neutral density) filter or a polariser (p. 970), as Metering explains, so following that advice with a filter on turns Auto into Normal.

Optional flash units will not fire (p. 521), and flash photography cannot be combined with HDR overlay (p. 966). Continuous release modes give one picture per press, and Bulb and Time are unavailable (p. 521). Nikon's exclusions (pp. 157, 521) are the auto shooting mode, photo flicker reduction and, once again, the other multi-frame tools: auto capture, pixel shift, focus shift, time-lapse movies, the interval timer, multiple exposures, bracketing and HSFC+. HDR overlay is unavailable under HLG (p. 462), Skin softening does not work with it (p. 480), and white balance bracketing cannot be used (p. 916). Unlike the other two, it is absent from the list of items a U position cannot store (p. 143).

04.Pixel Shift Shooting

Pixel shift shooting takes a series of NEFs, moving the image sensor between each, and Nikon's NX Studio software merges them on a computer into one high-resolution picture (p. 552). Nikon intends it for landscapes, buildings and other still subjects shot from a tripod (p. 553).

One Colour per Photosite

Each photosite sits under one colour filter in a repeating two-by-two block of one red, two greens and one blue, and demosaicing estimates the two colours each site missed, as RAW, JPEG, HEIF and Tone Mode shows. A red site knows its red exactly and borrows green and blue from neighbours. On a fine, regular pattern such as woven fabric, those guesses go wrong as false colour and moiré, a wavy interference pattern that is not in the subject.

Whole-Photosite Steps Fill In Colour

The photosites are about 0.00594 mm apart, 35.9 mm divided by 6,048, as Image Area and Crop works out. Shift the sensor by exactly that pitch and a scene point that sat under red falls under green. Shift down, then back across, and the point has been seen through red, green, blue and green, so nothing is borrowed. Nikon's pixel shift guide credits this with reducing moiré and colour fringing, because no colour is interpolated. The picture keeps 6,048 by 4,032 pixels, each holding measured colour.

Half-Photosite Steps Add Resolution

The guide says the sensor can also move by half a photosite, here about $0.00594 / 2 \approx 0.00297$ mm, which places samples between the original sites. When 16 or 32 shots merge into one picture, height and width both double, and the guide gives the FX figures.

$$6{,}048 \times 4{,}032 = 24{,}385{,}536 \qquad 12{,}096 \times 8{,}064 = 97{,}542{,}144 = 4 \times 24{,}385{,}536$$

Doubling each side multiplies the count by $2 \times 2 = 4$, about 97.5 million pixels. The RG says only that the merge makes a single high-resolution image (p. 552), and its specifications name pixel-shift photography (p. 1004) but give no merged size, so this size rests on the guide, which allows other image areas but sizes only FX.

What Each Shot Count Gives

The RG adds that a longer series takes longer and merges better (p. 552), and the default is 16 (p. 449). The guide recommends 32 unless there is reason not to, and 8 when brightness changes quickly, as at sunrise or sunset.

Delay, Interval, Options and Folders

Delay is the wait from the full press to the first frame (p. 552), in seconds (p. 555), 2 by default (p. 449). It lets the shake from your finger die away, and the guide suggests 2 to 5 seconds. The RG lists no other values.

Interval until next shot is the gap between frames in seconds (p. 552), 0 by default (p. 449). Nikon recommends 00 without flash (p. 555), and with flash a gap long enough to recharge (p. 552). The guide warns that uneven flash output can leave banding.

Thirty-two frames at a 2-second shutter speed need $32 \times 2 = 64$ seconds of exposure alone, plus the sensor's moves, whose duration Nikon does not state, and every second invites wind or cloud. The guide advises finishing fast, raising ISO, the sensitivity setting, if needed.

Options offers AE bracketing or Off, the default (pp. 449, 552). AE (auto exposure) bracketing varies exposure during pixel shift, with its own Number of shots and Increment, the exposure step, per sequence (p. 556), while ordinary bracketing stays restricted (p. 558). Starting storage folder offers New folder, a folder for each sequence, and Reset file numbering, restarting at 0001 with each new folder, both unticked by default (pp. 449, 552). Focus shift can also run pixel shift at each focus step (pp. 543, 548), as Focus Shift Shooting covers.

Shooting, Merging and the NEFX File

Before a run, Nikon recommends Pixel mapping from the setup menu, a test shot, and a fully charged battery or one of the power options it lists. Install the latest NX Studio, because older versions may not merge pixel shift files (p. 553).

Turning the mode on fixes Image quality at RAW and forces the electronic shutter whatever Custom Setting d6 Shutter type says (p. 554). A full press shows the shot count in the control panel and, after the delay, starts the NEFs (p. 557). On (series) stays ready until Off, and On (single photo) ends after one sequence. To stop partway, half-press the shutter-release button or press OK between shots (p. 557). With Beep at On, which First-Day Setup leaves off, a beep marks the end (p. 844).

The camera never merges the frames. NX Studio does (p. 557), and the guide says the merged file is a NEFX (.nefx), exportable as JPEG, HEIF (High Efficiency Image File Format) or TIFF (Tagged Image File Format, which nearly all editing software opens).

NX Studio's merge setting decides how many pictures a sequence yields: one, two, four or eight from 32 shots, one or four from 16, one or two from 8, and one from 4. Only pictures merged from 16 or more frames double in size. Suppose a gust shakes the tripod during frames 20 to 24 of 32. One picture uses every frame, shaken ones included, but two pictures of 16 leave the first built entirely from frames 1 to 16, still doubled, a rescue the guide describes.

Nikon gives no size for a sequence. If each frame matches a NEF at Lossless compression, about 27.3 MB (p. 1012), which Nikon does not confirm, 32 frames take $32 \times 27.3 \approx 870$ MB.

Cautions and Restrictions

A continuous release mode still yields just one photograph per press, a settings change partway can stop the run, and the Bulb and Time speeds are off the menu (p. 558). Focus locks to AF-S, single AF, which finds focus once and holds it, and any AF-area mode that only works under AF-C, continuous AF, falls back to Single-point AF. The d7 item Extended shutter speeds (M) stays OFF throughout (p. 558). Focus Modes has more.

Nikon's exclusions (pp. 157, 558) fall into three groups. Among timing aids, the self-timer and exposure delay mode are out. Among corrections, vibration reduction, photo flicker reduction and long exposure noise reduction are out. Every other multi-frame tool is out too, meaning bracketing, multiple exposures, HDR overlay, the interval timer, time-lapse movies, auto capture, HSFC+ and focus shift except through its own option, and so is video recording. Energy saving in photo mode is also out, and a U position cannot store pixel shift (p. 143).

With the self-timer, exposure delay mode and VR all ruled out, the tripod and Delay do all the steadying, though Nikon otherwise recommends Normal or Sport on a tripod, with Off possibly better on some (p. 498, Vibration Reduction). The forced electronic shutter limits flash sync to 1/60 s with no Auto FP (p. 1000), covered in Shutter Types, Rolling Shutter and Flicker. The guide lists what defeats a merge, matching the RG's warning (p. 557): moving subjects, heat haze, a camera nudged by wind or traffic, and changing light. It also says the HLG tone mode works with pixel shift, unlike the other two features.

05.Recommended Settings

All three items can join the i menu through Custom Setting f1 (p. 700), and Custom Setting f2 can put each on a button: hold it, turn the main command dial for the mode and the sub-command dial for the shot count, or HDR strength (p. 713). The i Menu and Custom Controls cover both.

Stage. With the Tamron 70-180mm f/2.8 I would try Lighten with two or three shots and Overlay shooting on, building a dancer's movement against the dark backdrop. A continuous release mode takes them in one burst (p. 516), which might turn one spin into a row of figures, an untested experiment.

Street. I would use Average with two shots, a face in one and a textured wall or reflection in the other.

High-contrast landscape. On the K&F Concept carbon-fibre tripod with the BH-36 head, I would take any ND filter off, meter with matrix and focus with AF-S and Single-point AF or Pinpoint AF. The saved NEFs allow a hand merge if halos appear.

Still object or still landscape. On the same tripod, with the Nikkor Z 85mm f/1.8 S for an object, I would run Pixel mapping and start with a full battery. In NX Studio I would merge to one picture, then to two or four if a gust spoiled part of the run.

06.Beginner Mistakes

Using Add over a lit background. Each frame adds its background. Use Lighten for bright subjects on black, Average when every frame is lit, or darken each frame by about $\log_2 N$ stops, my inference above, valid only if the camera adds linear readings.

Expecting a raw composite. Multiple exposure and HDR overlay both write a JPEG (pp. 514, 520). Set Save individual pictures (RAW) to ON before the first frame.

Wandering off mid-sequence. Idling in playback or the menus ends a multiple exposure (p. 516). Lengthen c3 Power off delay.

Leaving centre-weighted metering on for HDR. Auto strength quietly becomes Normal (p. 521). Switch to matrix, as Nikon recommends (p. 519).

Running pixel shift in wind or fading light. Movement or changing light spoils the merge (p. 557). Use 8 shots when the light is moving.

07.Related Chapters

• RAW, JPEG, HEIF and Tone Mode: the Bayer mosaic, 8-bit JPEG and the HLG limits.
• Image Area and Crop: sensor size, pixel pitch and the DX crop.
• Focus Shift Shooting: focus stacking, with pixel shift at each step.
• Metering: matrix, centre-weighted and metering through filters.
• Shutter Types, Rolling Shutter and Flicker: the electronic shutter pixel shift forces.
• Card Slots and File Handling: where composites and source NEFs land.