Choosing an Autofocus Setup

A heron stands motionless in the reeds for ten minutes, then lifts off and is across the river before you can swing the lens. That evening a dancer spins under one blue spotlight while three others cross in front of her. Both scenes defeat the camera's defaults, Single AF for Focus mode and Single-point AF for AF-area mode (p. 447), a pair built for a still subject in good light with nothing around it.

An autofocus setup is a set of answers to four questions. Does the subject move? How predictably? How much clutter surrounds it? How much light is there? Answer those and the settings follow nearly one for one. AF stands for autofocus throughout, and page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.Four Questions Behind Every Setup

Does the Subject Move

In Single AF, written AF-S, the camera focuses once and then holds that distance for as long as you keep focusing. In Continuous AF, written AF-C, it keeps correcting, which is what a moving subject needs (p. 121).

How much movement is too much for AF-S? Depth of field answers that. It is the band of distance around the focused point that still looks sharp, and for a subject well short of infinity it is close to

$$D \approx \frac{2 N c s^2}{f^2}$$

where $D$ is the depth, $N$ the f-number, $s$ the subject distance, $f$ the focal length and $c$ the largest blur spot that still passes for sharp, by convention 0.03 mm on a full-frame sensor, all in millimetres. For a bird 30 m away at 600 mm and f/6.3:

$$D \approx \frac{2 \times 6.3 \times 0.03 \times 30000^2}{600^2} = \frac{0.378 \times 9 \times 10^{8}}{3.6 \times 10^{5}} = 945\ \text{mm}$$

That is under a metre. A bird flying straight at you at an illustrative 10 m/s crosses it in $0.945 / 10 \approx 0.09$ s, so a distance locked by AF-S is wrong almost at once. Every setup below that involves something alive uses AF-C, even the perched one, because an animal at rest can lean or hop without warning.

How Predictably

Predictability decides how large the focus area should be and how stubbornly the camera clings to its subject. The three Dynamic-area AF sizes borrow from neighbouring points when the subject slips off one (p. 124). Wide-area AF (C1) and (C2) take a patch whose shape you set in points, from 1x1 to 21x13 in the full-frame FX image area (p. 125). 3D-tracking follows a subject you pick across the frame, and Auto-area AF lets the camera choose (p. 125).

The other half of predictability is what happens when something passes in front. Custom Setting a3 Focus tracking with lock-on sets how readily AF-C abandons the subject for an object crossing between it and the camera. Its Blocked shot AF response runs from 1 (Quick) to 5 (Delayed), with 3 as the default (pp. 623, 633). Higher holds the original subject through a passing branch, and lower lets focus move to the newcomer. Two exceptions shape the setups below: 3D-tracking always behaves as 3, and Auto-area AF treats 1 and 2 as 3 (p. 633).

How Much Clutter

Clutter is anything at another distance that could win the camera's attention: twigs in front of a bird, a crowd behind a busker, the dancer beside the one you want. Two tools fight it. A small area cannot be lured by a background it does not cover. Subject detection, in which the camera recognises people, animals, birds, vehicles or aircraft and places the point on them, down to an eye where it finds one, steers focus when the area is large (p. 127).

One restriction shapes every setup here. During autofocus, detection works only in the Wide-area AF modes, 3D-tracking and Auto-area AF (p. 128). Single-point AF, Pinpoint AF and all three Dynamic-area AF sizes ignore it. Nikon pairs Dynamic-area AF (L) with birds (p. 124), but that mode gives up eye detection, so I keep it for a bird flying in front of trees and otherwise use a Wide-area AF patch with Birds detection. In Wide-area AF (S) and (L) the camera favours the nearest subject in the area (p. 124), which helps against a far background and hurts against a near branch.

How Much Light

The Z6III focuses with a hybrid of phase detection and contrast detection (p. 1002), and both need light and contrast. Nikon rates the detection range at $-10$ to $+19$ EV, measured in AF-S at ISO 100 with an f/1.2 lens (p. 1002). EV, exposure value, is a brightness scale on which each step doubles the light, and ISO is the sensitivity setting.

Treat $-10$ EV as a best case. An f/2.8 lens passes $(1.2/2.8)^2 \approx 0.18$ of the light an f/1.2 lens does, about $\log_2 5.44 \approx 2.4$ stops less, and f/6.3 passes $(1.2/6.3)^2 \approx 0.036$, about $\log_2 27.6 \approx 4.8$ stops less. Nikon publishes no range for slower lenses, so those stops are my rough estimate, not a specification, and they hold only if the dark limit depends on nothing but the light reaching the sensor.

Nikon lists low contrast, night spotlights, lights that change brightness inside the point, and flicker under fluorescent or vapour lamps among the conditions that defeat autofocus (p. 122). Its remedy for low light is Custom Setting d11 Starlight view (photo Lv) at ON, photo mode only (pp. 122, 675), where Lv means live view, the picture the sensor feeds to the viewfinder and monitor. The a11 Built-in AF-assist illuminator lights the subject only in photo mode with AF-S, over roughly 1 to 3 m, and never in AF-C (pp. 643, 916).

Light also decides how far to trust the camera's judgement of focus. Custom Setting a1 AF-C priority selection offers Release, which fires whenever you press, Focus, which fires only once the camera judges the subject sharp, and Focus + release, which waits for focus on the first frame of a continuous burst when the subject is dark or low in contrast and lets the rest fire (p. 631). AF-S has a2 AF-S priority selection with Release and Focus (p. 632). The defaults are Release for a1 and Focus for a2 (p. 623).

02.The Setups

Each setup gives every focus choice, Custom Setting a6 AF activation included. All seven are for stills, since photo and video keep separate detection choices (p. 127) and video focus has its own part later.

Two habits run through them. In five of the seven I set a6 to AF-ON only, so a half-press of the shutter button no longer focuses and the AF-ON button does (p. 637). Back-Button Focus explains why that separation helps, and the two exceptions say why they keep the half-press. Wherever clutter sits at a known distance, I use the a15 Focus limiter setting added in Firmware 2.00, which confines autofocus to a range between 0.1 and 999 m that you set, with a Z mount lens attached (p. 648).

Perched Wildlife

A kingfisher on a branch over water, with the Nikkor Z 180-600mm f/5.6-6.3 VR, where VR is vibration reduction, Nikon's optical stabiliser. Continuous AF, since even a perched bird can hop (p. 121). Wide-area AF (C1) set to a small box, an illustrative 3x3, placed over the bird, which keeps most twigs out of the area while the mode still allows detection (p. 128). Birds detection, which puts the point on the bird's face, or on an eye when it finds one, and on the whole bird when it finds neither (p. 127).

a3 at 4, or 5 among swaying reeds, so a stem drifting across does not steal focus (p. 633). a6 at AF-ON only, which locks focus when I lift my thumb (p. 135), so I can focus and wait. Out-of-focus release at Enable, with a1 at Release to match. Disable would allow a frame only in focus, and Nikon does not say whether one still fires once my thumb is off and focus is held (p. 637), which is the whole point of the thumb here. a11 OFF, as Nikon advises near animals and birds (p. 129). With illustrative twigs at 4 m and the bird at 12 m, a range of 8 to 20 m puts the twigs out of bounds, and a button given the Focus limiter role switches it with one press (p. 705). The 180-600mm is not on Nikon's list of lenses the limiter does not support, which Firmware History reproduces. Nikon asks for the lens's own limit switch at FULL while a range is set, and a lens change disables the range until you set it again (p. 650).

Birds in Flight With the 180-600mm

Against open sky the job is keeping a fast bird inside the area. Continuous AF with Wide-area AF (L) and Birds: the large patch keeps a panned bird covered and detection puts the point on its face or eye (pp. 127-128). Once the camera has the bird, 3D-tracking with Birds is the alternative, following the bird I started on (p. 125). In front of trees I change to Dynamic-area AF (L) with no detection. Wide-area AF (L) favours the nearest subject in its area (p. 124), which in foliage is a branch, and 3D-tracking can lose a subject that matches its background in colour, brightness or pattern (p. 126). a3 at 3, raised to 4 when a flock or branches cross my bird, though 3D-tracking runs at 3 whatever is set (p. 633).

a1 at Release, because a slightly soft wingbeat is still a frame and a burst that pauses to confirm focus can miss the one I wanted (p. 631). This setup lives on user setting U1, with a6 at its default Shutter/AF-ON (p. 623). The U1 in Menus and Controls keeps a small area as its base and gives the sub-selector centre the plain AF-area mode role with a wide preset, here Wide-area AF (L), held for the flight. That role changes the area without starting focus (p. 705), so the half-press does the focusing throughout. Out-of-focus release belongs to AF-ON only and does not apply (p. 637). The lens's own limit switch at 6 m sets the near limit, so a15 stays OFF. Subject detection may perform worse during high-speed frame capture +, the release modes that shoot 15 to 120 frames a second (pp. 130, 156), so I prefer an ordinary continuous release mode unless I need that pace.

Stage Dance With the Tamron 70-180mm

A stage is the hardest scene here: dim light that changes colour and brightness with each cue, dancers crossing each other, and a black backdrop with nothing to focus on. With the Tamron 70-180mm f/2.8 Di III VC VXD G2 I use Continuous AF with Wide-area AF (C1) shaped as a tall box about the size of one dancer in my framing, with People detection to find her face and eyes (pp. 125, 127-128). Even for a solo I keep the box rather than 3D-tracking, which Nikon warns can lose a subject that changes colour or brightness (p. 126).

a3 at 4, because dancers pass in front of each other constantly (p. 633). a1 at Focus + release, so the first frame of each burst waits for focus on a dark dancer (p. 631). Here too a6 stays at its default Shutter/AF-ON (pp. 623, 637), since the Reference Guide does not describe Focus + release under AF-ON only. A button that toggles Starlight view (photo Lv) turns it on for a near blackout and off when the lights come up, as it can make the display slightly jerky (pp. 675, 711). Tone mode stays at SDR, standard dynamic range, because HLG, hybrid log-gamma, can lower detection performance (p. 130). A far limit just beyond the back wall would stop the lens chasing darkness. The Tamron is a Z mount lens, as the limiter requires (p. 648), but Nikon's list of lenses it does not support names only Nikkors, so check that it takes effect with the Tamron before relying on it.

Portraits With the 85mm f/1.8

A portrait has one subject and time to spare. With the Nikkor Z 85mm f/1.8 S I still choose Continuous AF, because at f/1.8 a sitter who leans a few centimetres leaves the sharp zone, as the diagnosis section works out. Nikon suggests Auto-area AF for portraits (p. 125), and with People detection it finds the eye. With two people in frame, the multi selector's left and right move between them (p. 128), and Touch Fn set to Switch eyes picks the eye with a thumb on the monitor (p. 730). I pick the nearer one. a3 at 3. a1 at Focus and Out-of-focus release at Disable, because a sitter moves little and leaves time to confirm focus. The limiter stays OFF, since nothing stands between me and the sitter.

Outdoor Dance

In daylight, movement takes over from light as the problem: leaps, spins and dancers who cross the frame faster than I can move a point. Continuous AF with 3D-tracking and People follows the dancer I start on and finds the face or eye (pp. 125, 128). a3 needs no thought, since 3D-tracking runs at 3. a1 at Release and Out-of-focus release at Enable, because bright light rarely leaves the camera lagging. Spins and flowing costumes change how large, what colour and how bright a dancer looks, conditions in which Nikon warns tracking may fail (p. 126), so Wide-area AF (L) with People is my fallback. The limiter stays OFF, because dancers move toward and away from me without pattern.

Street

Street pictures happen in a second, among crowds that are all clutter. Continuous AF with Auto-area AF and People finds a face anywhere in the frame (p. 125). a3 at 3, a1 at Release and Out-of-focus release at Enable, because a slightly soft picture of the moment beats a sharp one of the moment after.

The older method skips autofocus at the moment of shooting. Focus in advance at an illustrative 3 m and let depth of field cover the rest. A control given Save focus position stores that distance and one given Recall focus position returns the lens to it until the lens is detached (pp. 704, 718). Positions save in any focus mode, but on a zoom set the focal length first, since zooming afterwards shifts the recalled position (p. 718).

Landscape

A landscape does not move, and the question becomes exactly where the plane of sharpest focus falls. Single AF with Single-point AF, or Pinpoint AF for a smaller target such as the edge of a distant rock, which Nikon recommends for static subjects while warning that it can be slower (p. 123). Detection plays no part in either (p. 128). a2 stays at Focus, with Out-of-focus release at Disable to match (pp. 632, 637). With AF-ON only the distance holds after I let go (p. 135), so a bracket of exposures shares one focus. One trap: with Pinpoint AF, AF-ON only and Disable, the shutter fires whether or not the subject is sharp (p. 637).

At night I switch to manual focus, magnify the view and turn on a12 Focus peaking, which outlines sharp edges in colour (pp. 136-137, 645), with Starlight view (photo Lv) to brighten the display. The limiter stays OFF, since a landscape can run from a flower at my feet to the horizon.

03.Switching Between Two Setups

The perched kingfisher drops off the branch. You have perhaps half a second to go from a small box to a large area, and no menu is that fast. Most quick routes are roles set in Custom Setting f2 Custom controls (shooting).

The cleanest switch is the AF-area mode + AF-ON role. Holding a button with that role selects an AF-area mode chosen in advance and starts focusing, and letting go restores the previous mode (p. 705). AF-ON focuses with the small box on the perched bird, and a second button set to Wide-area AF (L) takes over the instant it flies. The Reference Guide does not say whether the role changes subject detection, so I keep Birds selected for both. The plain AF-area mode role makes the same swap without starting focus, and Cycle AF-area mode steps through a set of ticked modes with each press (p. 705).

The Fn2 button, the second function button, comes set to Focus mode/AF-area mode (p. 625): hold it and turn the main command dial for focus mode and the sub-command dial for AF-area mode (p. 713). A button with the Focus limiter role turns a15 on or off with a press (p. 705).

For a bigger change, Recall shooting functions applies a stored group of still settings while its button is held, and Recall shooting functions (hold) applies them with one press and restores the previous ones with the next (pp. 707-708). Both work only in modes P, S, A and M, and only the first can store AF-ON (pp. 707-708). I keep focus out of the group, as Recall Shooting Functions explains.

The largest switch is a user setting. The U1, U2 and U3 positions on the mode dial each hold most of the camera's settings, Custom Settings included, though Nikon lists a few they cannot store, saved with Save user settings in the setup menu (pp. 142-143, 810). They suit a change of venue, theatre to river, more than a change mid-scene.

04.Diagnosing a Missed Focus

Most missed focus traces back to a setting that answered one of the four questions wrongly. Start from the symptom.

Focus Lands on the Background

The subject was smaller than the focus area, or detection did not find it, so the camera chose whatever behind it had more contrast. A subject smaller than the focus point is on Nikon's list of failures (p. 913). Shrink the area to a smaller box or Single-point AF, as AF-Area Modes describes. If detection is on and still misses, the face may be too small in the frame (p. 129), so move closer or zoom in. A far background the lens keeps racking to can be fenced off with a far limit in a15 (p. 648).

Focus Jumps Between Subjects

A low a3 value lets focus leave for anything that crosses (p. 633), so raise it, as The Focus Custom Settings explains. In Auto-area AF the camera may simply prefer another face: press left or right to choose (p. 128), or use a box that holds one person. Auto detection looks for people, animals and vehicles together (p. 128), so pick the one class you want, as Subject Detection shows.

The Lens Hunts in the Dark

On a dark subject with no contrast the lens racks back and forth without settling (p. 122). In AF-C a flashing focus point means the camera has given up: release the button and press again (p. 913). Turn on Starlight view (photo Lv) (p. 122), aim the point at an edge with contrast, and, in a continuous release mode with a6 at Shutter/AF-ON, set a1 to Focus + release so the first frame of each burst waits (p. 631). For a still subject within about 1 to 3 m in AF-S the AF-assist lamp helps (p. 643), though not near animals (p. 129).

Sharp Eyes, Soft Nose

This is depth of field, not autofocus. The formula from the start of the chapter gives, for the 85mm at f/1.8 and 1.5 m,

$$D \approx \frac{2 \times 1.8 \times 0.03 \times 1500^2}{85^2} = \frac{0.108 \times 2\,250\,000}{7225} \approx 33.6\ \text{mm}$$

about 3.4 cm in total, split nearly evenly around the eye. The exact thin-lens figures put a little less in front, about 1.6 cm, so a nose further forward has started to soften. Stopping down to f/4 raises the depth in proportion to the f-number, to $33.6 \times 4 / 1.8 \approx 74.7$ mm. Stepping back to 2 m at f/1.8 gives $0.108 \times 4\,000\,000 / 7225 \approx 59.8$ mm. Switch eyes to the nearer eye (p. 705) puts the sharp band where it does most good.

The Shutter Will Not Fire

With a6 at Shutter/AF-ON, an a1 or a2 at Focus holds the shutter until the camera judges the subject sharp (pp. 631-632). With AF-ON only, Disable does the same, except in Pinpoint AF (p. 637), and since Nikon does not say how it combines with a1 and a2, a Focus there may hold it too. Either way the setting is doing its job. If the frame matters more than certainty, choose Release, as Back-Button Focus discusses.

05.Recommended Settings

Across all seven, a11 goes from its default ON to OFF (p. 623) except for a close, still subject in the dark, a15 stays OFF until a scene offers a known distance to fence, and a spare button carries AF-area mode + AF-ON, except in flight, where the sub-selector centre is plain AF-area mode.

06.Beginner Mistakes

Leaving Single AF on for moving subjects. It is the default (p. 447), and it locks a distance the subject has already left. Use Continuous AF for anything alive.

Expecting eye detection in Dynamic-area AF. Detection works only in the wide-area, 3D-tracking and auto-area modes (p. 128). When you want a bird's eye, use a Wide-area AF box with Birds.

Forgetting the limiter is on. A lens that refuses a near subject may be fenced out by a15. Look for the limiter icon in the display (p. 648) and keep a Focus limiter button to switch it off (p. 705).

07.Related Chapters

• How Autofocus Works on the Sensor: why light and contrast limit focus.
• Focus Modes: AF-S, AF-C and manual focus.
• AF-Area Modes: every area mode in full.
• Subject Detection: the detection classes.
• Back-Button Focus: a6 and the AF-ON button.
• The Focus Custom Settings: the a group in full.
• Focus Limiter, Saved Positions and Manual Focus: a15, saved positions and peaking.