Focus Shift Shooting

A lichen-covered rock sits half a metre in front of the tripod and a ridge stands on the horizon. With the Nikkor Z 24-70mm f/4 S at 24 mm and f/8, focus on the rock and the ridge goes soft. Focus on the ridge and the rock does. Closing the aperture to f/22 puts both inside the sharp zone on paper, and then blurs every point in the picture, the focused ones included, by an amount comparable to what the sharpness standard allows.

Focus shift shooting takes a different route. The camera fires a short run of frames and moves focus a little farther away for each one. Software on a computer later keeps the sharpest part of every frame and builds one picture that is sharp from the rock to the ridge. How many frames that takes is a matter of arithmetic, and the camera's part is smaller than many expect.

Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.Why Only One Distance Is Sharp

A lens bends the light from each point of the subject so that it meets at a point again behind the lens, and the nearer the subject, the farther back that meeting point lies. The sensor is one flat plane, so only one subject distance lands on it as a point. Light from every other distance arrives before it has converged or after it has crossed, and records as a small disc.

Put numbers on the opening scene, treating the lens as one thin piece of glass. Focused at 0.5 m, a 24 mm lens sits about 25.2 mm in front of the sensor, while light from the distant ridge converges 24 mm behind the lens, some 1.2 mm short of the sensor. The f-number $N$ is the focal length divided by the width of the opening, so at f/8 the opening is $24/8 = 3$ mm wide. That 3 mm cone narrows to a point over 24 mm and spreads again for 1.2 mm, so it reaches the sensor as a disc about $3 \times 1.2/24 = 0.15$ mm across. With $f$ the focal length and $s$ the focus distance, the disc left by a point at infinity is

$$b \approx \frac{f^2}{N\,(s-f)} = \frac{0.024^2}{8 \times 0.476} \approx 0.000151\ \text{m}.$$

A disc small enough still looks like a point. The largest disc that passes for one is the circle of confusion, written $c$, and the band of distances whose discs stay under it is the depth of field, the zone in front of and behind the focus distance that still looks sharp. For $c$ this guide takes 0.03 mm, a common convention for a 36 by 24 mm frame and not a Nikon figure, as Focus Modes does. The ridge's 0.15 mm disc is five times that, so the ridge looks soft.

The Sharp Zone in Dioptres

One distance governs everything here: the hyperfocal distance $H$, the nearest focus distance whose sharp zone reaches infinity.

$$H \approx \frac{f^2}{N\,c} = \frac{0.024^2}{8 \times 0.00003} = 2.4\ \text{m}.$$

Focus at 2.4 m and the zone runs from half of it, 1.2 m, to infinity, which leaves the rock at 0.5 m out.

The arithmetic gets simpler in dioptres, one divided by the distance in metres, the same ruler Focus Limiter, Saved Positions and Manual Focus uses for the focus range. In dioptres the edges of the sharp zone sit at

$$\frac{1}{s_{\text{near}}} \approx \frac{1}{s} + \frac{1}{H}, \qquad \frac{1}{s_{\text{far}}} \approx \frac{1}{s} - \frac{1}{H}.$$

Focused on the rock, $1/s = 2$ dioptres and $1/H = 0.42$, so the zone spans 2.42 to 1.58 dioptres, or 0.41 m to 0.63 m. The total, 0.22 m, agrees with the formula in Focus Modes, which gives 0.21 m.

While focus is many focal lengths away, the zone is $2/H$ dioptres wide wherever you focus: $2/2.4 = 0.83$ dioptres at 24 mm and f/8. In metres the zone looks tiny up close and huge far away, but in dioptres every frame covers the same width.

02.The Blur That Stopping Down Adds

The obvious cure is a smaller opening. Nikon lists f/22 as the smallest aperture of the 24-70mm f/4 S on its product page. There $H = 0.024^2/(22 \times 0.00003) \approx 0.87$ m, and focused at that distance the zone reaches from 0.44 m to infinity, rock and ridge together.

The cost is diffraction. Light passing a small opening spreads, so even a perfectly focused point becomes a disc, and the disc grows as the opening shrinks. Picture Controls and Colour works the formula: for green light of wavelength $\lambda = 0.55$ micrometres the disc measures about $d \approx 2.44\,\lambda\,N$ across, and one Z6III pixel is about 5.94 micrometres wide.

At f/22 the disc is $2.44 \times 0.55 \times 22 \approx 29.5$ micrometres, comparable to the 30-micrometre circle of confusion, so the point you focused on is already near the limit before any defocus is added. Past a certain aperture the depth of field grows on paper while fine detail fades everywhere. Nikon's tip for this feature is to stay wider than f/8 to f/11, because very small apertures can cost definition (p. 551).

Diffraction compensation, ON by default in the photo shooting menu, is meant to reduce this softening (pp. 447, 478). Nikon does not say how, and no processing restores detail the optics never delivered, so I treat it as a help, not a licence for f/22.

03.Focus Stacking

Since each frame is sharp over a band $2/H$ dioptres wide, a run of frames whose bands touch or overlap can cover any range. Merging software compares the frames area by area, keeps whichever is sharpest at each spot and blends the result into one picture. That process is focus stacking. With $s_1$ the nearest distance that must be sharp and $s_2$ the farthest, you need at least

$$n \ge \frac{H}{2}\left(\frac{1}{s_1} - \frac{1}{s_2}\right)$$

frames, rounded up, with $1/s_2 = 0$ when the far limit is infinity. The figures below are illustrative, worked with $c = 0.03$ mm.

The opening scene at 24 mm and f/8 needs $1.2 \times 2 = 2.4$ frames, so three at minimum. I would shoot four or five, so that neighbouring bands overlap and a small error leaves no soft strip. At 70 mm and f/8, $H = 0.07^2/(8 \times 0.00003) \approx 20.4$ m, and a foreground at 1 m needs $10.2 \times 1 = 10.2$, so eleven frames. The band narrows with the square of the focal length.

Two effects sit outside the arithmetic. Anything that moves between frames, such as grass in wind, appears in different places, and the merge has to pick one version. And refocusing changes the framing slightly, an effect called focus breathing, so the merge must line the frames up before comparing them.

What the Camera Does Not Do

The Z6III does not merge the frames. Nikon describes them as photos to be combined later by focus stacking (p. 542), and the focus shift pages name no in-camera merge and no software for the job. Each frame is an ordinary picture at your image quality setting. A lossless NEF, Nikon's raw format, is about 27.3 MB in the full-frame FX image area under the conditions Nikon lists (p. 1012), so a twelve-frame stack takes about $12 \times 27.3 \approx 328$ MB of card.

You will also find no stack preview, no map of the areas that will come out sharp, and no Exposure smoothing item, which Interval timer shooting and Time-lapse video carry (p. 448). Its nearest relative here is First-frame exposure lock.

04.The Focus Shift Shooting Menu

Before using it Nikon asks for autofocus (AF) in AF-S, Single AF, or AF-C, Continuous AF, and for any release mode except the self-timer and HSFC+, the high-speed frame capture + modes (p. 542). Single AF and Single frame are the stills defaults, so a camera at its defaults is ready (p. 447).

Number of Shots

No. of shots goes up to 300 (pp. 542, 545). A run stops at that count or when focus reaches infinity, whichever comes first (pp. 549-550), so a high count costs nothing on a landscape. On a tabletop the frames focused behind the object are waste, so I set the arithmetic's answer plus a few. Nikon's advice is to be generous and discard extras, with over 100 frames for insects and a handful for a wide landscape (p. 545).

Focus Step Width

Focus step width sets how far focus moves between frames, smaller to the left and larger to the right (p. 545). Nikon warns that large values raise the risk of unsharp areas in the stack, recommends 5 or less, suggests trial runs, and advises smaller steps and more frames close up, where depth of field shrinks (pp. 545, 551).

Nikon gives no unit for the step and does not say whether it scales with aperture or focal length, so the dioptre arithmetic says how many bands you need, not which step value delivers them. The practical check is a trial run: open neighbouring frames in playback at high magnification and confirm that detail sharp in one is still sharp in the next.

Interval Until Next Shot

Interval until next shot is a pause in seconds (p. 542), and 00 is the recommendation without flash (p. 546). With a flash the interval must outlast the recharge, or the flash may fire short of full power, and at 00 it does not fire at all, whatever the Flash mode setting (p. 550). Shutter speed and recording time can vary, so the camera may not hold the interval exactly (p. 550).

With $n$ frames and an interval $t$ there are $n - 1$ pauses, plus a start delay of about 3 s (p. 549), plus the exposures. Nikon does not say whether the interval counts from the start of one frame or the end of its recording, so treat this as a floor. An illustrative twelve-frame tabletop stack with a 3 s interval for flash recharge takes at least $3 + 11 \times 3 = 36$ s.

First-Frame Exposure Lock

With First-frame exposure lock at ON, every frame uses the exposure of the first. At OFF the camera sets exposure afresh before each frame (p. 542). Nikon suggests OFF when the light will hold steady and ON for landscapes under changing light, then adds that large brightness changes during a run can still look like exposure variation under ON, which OFF addresses (p. 546).

My reading, which Nikon does not state, is that the lock only matters when the camera picks part of the exposure: in P, S or A, the modes where it sets shutter speed, aperture or both, or with Auto ISO choosing the ISO, the sensitivity setting. In M, manual exposure, at a fixed ISO it has nothing to change. I shoot stacks that way and leave the lock ON.

Electronic Shutter and Volume

The Electronic shutter options item has two parts. Leave Electronic shutter at its default ON and every frame of the run is taken electronically, overriding Custom Setting d6 Shutter type (pp. 448, 542). I read OFF as handing the choice back to d6, though Nikon does not say so. Shutter Types, Rolling Shutter and Flicker covers what the electronic shutter costs, including banding under some artificial light. For flash it brings two more limits: sync at 1/60 s or slower, and no Auto FP high-speed sync, the mode that lets a flash work at faster speeds (p. 1000).

Volume sets the electronic shutter's sound, and its default is 0 (p. 448). Any other value makes every frame audible, and Silent mode in the setup menu does not mute it (p. 550). Nikon rules out flash photography in silent mode and says to set it to OFF before using any flash (pp. 415, 966). Silent mode forces the electronic shutter, mutes the beep speaker and stops a flash unit mounted on the camera from firing (p. 846). So before any flash stack I turn Silent mode off with the Illuminator button, which carries it in this guide's setup. The beep, when On, sounds as a run ends, but its default is Off (pp. 806, 844).

Auto Reset Focus Position

With Auto reset focus position at ON, focus returns to the starting point when a run ends, ready for a repeat. At OFF, the default, focus stays where the last frame left it, so a second run carries on from there (pp. 448, 543, 547). I set ON, so a gust or a change of light costs only a second press of Start.

Options and Pixel Shift

Options is Off by default (p. 448). Its one alternative, Pixel shift shooting, takes a whole pixel shift series at every focus step, and pixel shift then carries its own shot count and its own pause. The focus shift interval stops applying, autofocus is fixed at AF-S, and an area mode available only in AF-C reverts to single-point AF (pp. 543, 548). The run starts after about 5 s instead of 3 (p. 549). Firmware History records the combination as new in 2.00, and Multiple Exposure, HDR Overlay and Pixel Shift explains pixel shift itself.

Frames multiply. Eleven focus steps with $k$ pixel shift shots each make $11k$ frames. Nikon gives no size for these. If each matches the 27.3 MB lossless NEF above, which Nikon does not confirm, my estimate is $300k$ MB. Nikon names NX Studio for merging pixel shift (p. 552), but the focus shift pages do not say how to run both merges on one set. Outdoors, where wind moves things, I leave Options at Off.

Starting Storage Folder

Starting storage folder has two tick boxes, both clear by default (p. 448). New folder opens a fresh folder for each run, and Reset file numbering restarts names at 0001 whenever that happens (p. 543). I tick New folder so each stack lands in its own folder, and leave Reset file numbering clear so names stay unique across stacks. Folders stop at 999, as Card Slots and File Handling explains.

05.Running a Sequence

Take a test shot at your settings first, and start with a full battery or mains power, since the camera stays awake for the whole run (pp. 544, 550).

Then set the start point. The camera works from the focus you set toward infinity, so place focus a touch nearer than the closest thing that must be sharp, then leave the camera alone (p. 544). For landscapes I use AF-S with Single-point AF or Pinpoint AF, as in Choosing an Autofocus Setup, and press AF-ON on the ground just in front of the rock.

Open Focus shift shooting, check the settings, highlight Start and press OK. An icon flashes in the control panel and the run begins about 3 s later, and the monitor goes dark while it runs (p. 549). I expect that delay to let any shake from pressing OK die away, which matters because the self-timer is excluded. The top control panel shows the focus shift indicator and the shots left (pp. 942-943).

A run can be cut short in three ways: pick Off for the menu item, give the shutter-release button a half-press, or press OK in a gap between frames (p. 549). Adjusting other settings mid-run may stop it, and with the shutter speed at Bulb or Time the camera shows a warning instead of starting (p. 550). The item can be placed in the i menu (p. 700). U1, U2 and U3 cannot hold it (p. 143), so, as User Settings and Settings Files puts it, set it up after you turn the dial.

06.What It Will Not Combine With

Nikon lists what cannot run alongside focus shift, and other features name it in turn. In the table, MF is manual focus and HDR is high dynamic range.

Nikon introduces its list with "including", so it may not be complete (p. 550). The focus shift pages name no lens restriction and no image-area restriction (pp. 542-551). Because the feature needs AF-S or AF-C, I expect it to need a lens the camera can autofocus, but that is my inference.

07.Landscapes and Small Objects at Home

Landscape. The 24-70mm f/4 S goes on the K&F Concept carbon-fibre tripod with the BH-36 ball head. At 24 mm and f/8, four or five frames cover a foreground at half a metre, provided the wind drops. A variable neutral density (ND) filter, which cuts light evenly across colours, lengthens each frame. Seven stops, the most I use at 24 mm for reasons Long Exposure and Night explains, multiply the exposure by $2^7 = 128$, so 1/60 s becomes about 2.1 s, and an illustrative five-frame run takes at least $5 \times 2.13 + 3 \approx 13.7$ s.

Small objects at home. Nikon lists the lens's minimum focus distance as 0.3 m and its maximum reproduction ratio, image size over subject size, as $0.3\times$ on its product page. The sensor is 35.9 mm wide (p. 997), so the smallest subject that fills the frame's width is $35.9/0.3 \approx 120$ mm across. A teapot or a bunch of flowers can fill the frame. A coin cannot, and cropping it costs pixels. Without a macro lens, the insect stacks of more than 100 frames that Nikon mentions (p. 545) are out of reach.

Take an illustrative object 0.35 m to 0.45 m from the lens, not the sensor, at 70 mm and f/5.6. Then $H = 0.07^2/(5.6 \times 0.00003) \approx 29.2$ m, the span is $1/0.35 - 1/0.45 \approx 0.635$ dioptres, and the formula asks for $14.6 \times 0.635 \approx 9.3$ frames. Up close the formula flatters, because it assumes focus many focal lengths away. At 0.35 m a 70 mm lens is only five focal lengths away, which shrinks the real depth by about $(0.35 - 0.07)/0.35 = 0.8$, so the count rises to $9.3/0.8 \approx 11.6$. A zoom is not one thin lens, so treat twelve frames as a guide, not a prediction.

For light I use the Godox V100N through the X3 Pro-N trigger, with the interval set longer than its recharge. Nikon does not say whether the 00 rule holds through a radio trigger, so I treat that as an open question.

08.Recommended Settings

Wildlife, stage and portraits. I do not use focus shift for these, since no merge can reconcile a bird or a dancer in two places.

09.Beginner Mistakes

Starting on the subject instead of in front of it. The run only moves away from the camera, so focus set exactly on the rock leaves its nearest edge soft. Start a little nearer (p. 544).

Stopping down to f/16 or f/22 to be safe. At f/22 the diffraction disc is comparable to the circle of confusion everywhere in the frame. Stay at f/8 or wider and add frames instead (p. 551).

A large step to save time. Gaps between sharp zones show as soft strips after the merge. Keep to 5 or less and check neighbouring frames in playback (p. 545).

Leaving the interval at 00 with a flash. The flash will not fire at all. Set an interval longer than its recharge (p. 550).

Expecting a finished picture. The camera records separate frames, and the merge happens later on a computer (p. 542).

10.Related Chapters

• Focus Modes: AF-S, AF-C and the depth-of-field formula.
• Focus Limiter, Saved Positions and Manual Focus: the dioptre ruler and manual focusing.
• Shutter Types, Rolling Shutter and Flicker: what the electronic shutter costs.
• Release Modes, Frame Rates and the Buffer: Single frame and the HSFC+ limits.
• Silent Shooting, Timers and Time-Lapse: silent mode and the interval timer.
• Picture Controls and Colour: diffraction and Diffraction compensation.
• User Settings and Settings Files: why U1 to U3 skip this item.
• Multiple Exposure, HDR Overlay and Pixel Shift: pixel shift on its own.
• Flash: the Godox kit, sync speeds and recharge.