AF-Area Modes

A wren sings from the middle of a hawthorn twelve metres away, and a dozen twigs cross the space between you. The Nikkor Z 180-600mm f/5.6-6.3 VR is at 600 mm with a wide focus area over the bird. The camera focuses, the twig across the wren's chest comes out crisp, and the wren is soft. Nothing broke. The wide area saw two things at different distances and, as Nikon designed it, picked the nearer one (p. 124). Shrink the area to one small point on the eye and the next frame is sharp. Then the wren flies, and the small point is left holding empty sky.

The Z6III offers eleven ways in photo mode, and seven in video, to decide which part of the frame the camera consults when it focuses (pp. 495, 605, 1002). Each sits somewhere on that same trade: a small area can slip off a moving subject, and a large one can grab the wrong thing. Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.What an AF-Area Mode Decides

Where, Not When

Autofocus, or AF, is the camera moving the lens's focusing elements for you, and two settings steer it. The focus mode says when: AF-S (single-servo AF, Single AF in the menus) focuses once and locks, AF-C (continuous-servo AF, Continuous AF) keeps adjusting while you hold the button, and AF-F (full-time AF) refocuses on its own in video only (p. 121). The AF-area mode says where. Focus Modes covers the first question and this chapter the second.

The camera measures focus on the sensor itself, with hybrid phase-detection and contrast AF (p. 1002), two ways of judging which way focus is off and when detail is crisp, which How Autofocus Works on the Sensor takes apart. What matters here is that measurements come from patches of the frame called focus points, and the AF-area mode chooses which patches count.

With the FX image area, meaning the whole full-frame sensor, photo mode has 273 focus points in single-point AF and 299 in auto-area AF (p. 1002). Now 273 is $21 \times 13$, and the largest custom box the camera allows is exactly $21 \times 13$ points (p. 125). Nikon never says the points form a grid 21 wide and 13 high, but both figures fit that reading, and this chapter uses it as a working picture.

One Distance per Frame

A lens focuses at one distance. Things a little nearer or farther still look sharp, and the depth of that zone is the depth of field, DoF. When the subject is much closer than the hyperfocal distance $H = f^2 / (N c)$, the focus distance at which the sharp zone just stretches to infinity, a good approximation is

$$\text{DoF} \approx \frac{2 N c d^2}{f^2}$$

where $N$ is the f-number, $d$ the subject distance, $f$ the focal length and $c$ the circle of confusion, the largest blur spot that still passes for a point. For full frame, $c = 0.03$ mm is the customary convention, not a Nikon figure.

For the wren, in metres, $f = 0.6$, $N = 6.3$, $c = 0.00003$ and $d = 12$. Then $H = 0.36 / 0.000189 \approx 1{,}905$ m, so 12 m is comfortably small, and

$$\text{DoF} \approx \frac{2 \times 6.3 \times 0.00003 \times 12^2}{0.6^2} = \frac{0.0544}{0.36} \approx 0.15 \text{ m}$$

About 15 cm of sharpness, roughly 7.5 cm either side. A twig 30 cm in front of the wren is outside it, and so is the wren if the camera focuses on the twig. Whatever the area mode hands to the focusing system comes out sharp.

Slip and Grab

A small area fails by slipping. Suppose a bird crosses the frame in two seconds. On the 21-column picture its image moves about 10.5 points a second, so a pan running 10% slow lets it drift about one point a second from where you aimed. A single point has no margin and soon sits on sky. A $5 \times 5$ box centred on the bird has two points of margin each way, about two seconds of grace. The numbers are illustrative.

A large area fails by grabbing. Suppose that around the bird one point in ten falls on a nearer twig, at random. The chance that a box of $n$ points holds no twig is $0.9^n$: 0.9 for one point, $0.9^{9} \approx 0.39$ for $3 \times 3$, $0.9^{25} \approx 0.07$ for $5 \times 5$ and $0.9^{81} \approx 0.0002$ for $9 \times 9$. The count of points grows with the square of the width, so doubling the width roughly quadruples the places an intruder can hide. The 10% is invented. The squaring is not.

The stage shows both failures in one evening. A dancer is 15 m away, with the Tamron 70-180mm f/2.8 Di III VC VXD G2 at 180 mm and f/2.8. The formula gives $2 \times 2.8 \times 0.00003 \times 15^2 / 0.18^2 \approx 1.17$ m, about 58 cm either side. Put a single point on her sternum, she turns, the point lands on the brightly lit backdrop five metres behind, and the lens racks to the scenery. A wide area keeps her, because she is nearer than the backdrop. Then a second dancer crosses downstage at 13 m, two metres in front and far outside that margin, and the wide area hands focus to him.

02.The Modes One by One

The photo item starts at Single-point AF (p. 447) and lists eleven modes. The i menu, the quick-settings screen behind the i button, offers the same choice as AF-area mode/subj. detection (p. 123).

Video has its own item, also starting at Single-point AF (p. 579). Pinpoint AF, the three Dynamic-area AF sizes and 3D-tracking are photo-only, and Subject-tracking AF is video-only (pp. 123-125, 1002). The two items are separate, so a stills choice does not follow you into filming.

"Any" means the Reference Guide states no focus-mode restriction. The figure below draws the shapes on one frame.

Pinpoint AF

Pinpoint AF uses a point smaller than the single-point one, for a precise spot, and Nikon warns that focusing may be slower (p. 123). It suits static subjects such as buildings, studio products and close-ups, and works only in photo mode with Single AF (p. 123). One trap involves Custom Setting a6 AF activation. At AF-ON only, a half-press no longer focuses and the AF-ON button on the back does instead. Its Out-of-focus release option at Disable normally holds the shutter until the camera has focus, yet with Pinpoint AF the shutter can fire regardless (p. 637).

Single-Point AF

Single-point AF focuses on the point you place, and Nikon pitches it at stationary subjects (p. 123). If the subject leaves the point, the camera focuses on whatever is now under it. It is the default in both menus and the one mode the a8 and g4 limits will not let you remove (pp. 447, 579, 623, 628). It threads focus through a gap in branches, or puts it on one chosen rock in a landscape.

Dynamic-Area AF

Dynamic-area AF (S), (M) and (L) focus on your point, and if the subject briefly leaves it the camera uses information from the surrounding points (p. 124). Your point keeps priority and the neighbours are a safety net. That is the real difference from the wide-area modes, where the nearest subject anywhere in the area wins. All three need photo mode and Continuous AF (p. 124).

Nikon ties the size to how predictable the subject is (p. 124). S suits predictable paths, such as runners or cars on a track. M suits erratic movement, such as football players. L is for fast subjects that are hard to keep in one point, and Nikon's example is birds. The Reference Guide gives no neighbour counts, but with a10 Dynamic-area AF assist at its default ON, the display draws the surrounding points, so the viewfinder shows the size (pp. 623, 641).

Wide-Area AF (S) and (L)

Wide-area AF (S) and Wide-area AF (L) work like single-point AF over a larger area, (L) being the larger (p. 124). Nikon recommends them for snapshots and moving subjects, and in video for smooth focus during pans, tilts and moving subjects. If the area holds subjects at different distances, the nearest gets priority (p. 124), the rule that caught the wren. Subject detection works in both (p. 128), and the Reference Guide gives no sizes for either. Video's Hi-Res Zoom, which zooms electronically without a zoom lens, fixes the mode at Wide-area AF (L) and hides the point (p. 217).

Custom Wide-Area Boxes

Wide-area AF (C1) and Wide-area AF (C2) are wide areas whose width and height you choose, in focus points, for when you can predict the shape you need (p. 125). Select C1 or C2 and press and hold OK. The camera asks for a size: up and down on the multi selector, the ring around OK that you press in four directions, set the height, left and right the width (p. 125). With FX, the photo menu offers 77 sizes from $1 \times 1$ to $21 \times 13$, and the video menu 66 from $1 \times 1$ to $21 \times 11$. Other image areas offer different sizes (p. 125).

The counts carry a hint. 77 is $11 \times 7$, exactly the combinations of odd widths from 1 to 21 and odd heights from 1 to 13, and 66 is $11 \times 6$, the same with heights up to 11. That suggests the box grows two points at a time around a centre point. Nikon prints no list, so treat this as arithmetic and read the size off the display. A $9 \times 5$ box covers 45 points, $45 / 273 \approx 16.5\%$ of the grid.

3D-Tracking

3D-tracking starts from the point you place. Press AF-ON or half-press the shutter-release button and focus follows the subject through the frame. Let go and your original point returns (p. 125). If the subject leaves the frame, lift your finger, recompose with the subject on the point and start again. It needs photo mode and Continuous AF (p. 125), and it uses subject detection (p. 128). Pixel shift shooting, which fixes focus at AF-S, swaps this and the dynamic-area modes for Single-point AF (p. 558).

The a3 Blocked shot AF response, which sets how readily focus jumps to something crossing in front, is fixed at 3 here whatever you choose (p. 633). a10 sets the tracking point's colour, White by default or Red (pp. 623, 642). With f5 Focus-point lock on, the point still tracks during a half-press (p. 732). Nikon lists what can defeat tracking, among them subjects that blend into the background, change in size, colour or brightness, move fast or are hidden (p. 126). Stage light that shifts colour mid-phrase is on that list.

Subject-Tracking AF

Subject-tracking AF is the video counterpart. Place the point over the subject, start tracking with OK, AF-ON or a half-press, and press OK again to stop and return to the centre point (p. 125). The same cautions apply (p. 126). With the touch screen set to Move focus point or Touch AF, a tap on a subject starts tracking it (pp. 132-133).

Auto-Area AF

Auto-area AF hands the choice to the camera, which detects the subject and picks the point, from 299 positions in photo mode with FX (pp. 125, 1002). Nikon recommends it when there is no time to place a point, for portraits and for snapshots (p. 125). When it finds several subjects of the chosen type it marks each with a grey point and flags its own choice, and left and right on the multi selector move focus to another (p. 128).

Handing over the point has side effects. You cannot move it yourself (pp. 131, 914). Spot metering, normally centred on the focus point, reads the centre point instead (p. 487). Nikon says to choose another mode for focus lock (p. 134), and f5 Focus-point lock does nothing here (p. 732). Link VR to focus point aims stabilisation at the frame's centre when several points are shown (p. 499), and an a3 setting of 1 or 2 behaves as 3 (p. 633).

03.Moving and Recentring the Point

In every mode except Auto-area AF you place the point (p. 131). The multi selector moves it while the standby timer, which keeps the displays and meters awake after you touch a control, is running. OK jumps it back to the centre, that button's default role (pp. 131, 626). The sub-selector, the small joystick on the back, can move it instead, and its centre locks focus and exposure by default or can take Select center focus point (pp. 131, 626).

The Focus Custom Settings covers the settings that shape this. a4 Focus points used at Alternating points uses about a quarter of the positions, so each press travels farther, and leaves Pinpoint AF alone (pp. 131, 634). a9 lets the point wrap from one edge to the opposite and is OFF by default (pp. 623, 640). a14 sets how fast a held selector moves it, Normal by default (pp. 623, 647). a5 can store separate points for horizontal and vertical framing (p. 635).

04.Switching Modes Quickly

Out of the box, hold Fn2 and turn the sub-command dial at the front of the grip to step through AF-area modes, while the main command dial changes focus mode (pp. 74, 625, 713). The f2 Command dials entry has Focus/AF-area mode selection to swap the dials (p. 716).

Eleven modes is a long dial in a dark theatre. a8 lets you untick the ones you never use, so the sub-command dial offers only the rest when you hold a button set to Focus mode/AF-area mode (p. 639). Every mode starts ticked and Single-point AF cannot be removed (p. 623). Video has its own copy, g4 (pp. 628, 759). I keep seven ticked, those my recommendations use, from widest to narrowest: Auto-area AF, Wide-area AF (L), 3D-tracking, Wide-area AF (C1), Dynamic-area AF (L), Single-point AF and Pinpoint AF. No mode is then more than six clicks from another, assuming the dial does not wrap, which the Reference Guide never says. For a stage evening I trim the list to two or three, as Menus and Controls does.

f2 can also give a button one of three roles (p. 705). Cycle AF-area mode steps through a list you choose. AF-area mode switches to a preset mode while held and back on release. AF-area mode + AF-ON does that and starts autofocus too, which suits back-button focusing. Put AF-area mode on the sub-selector centre with f16 Prefer sub-selector center at OFF, and the sub-selector still steers the point while you hold its centre (p. 744). Custom Controls covers assignment, and Menus and Controls shows the bird setup.

a7 Focus point persistence affects those two hold-to-switch roles (p. 638). If you half-press and switch from a mode where the camera picks the point, such as Auto-area AF, to one where you pick it, Auto, the default, keeps the camera's last point, and Off restores yours (pp. 623, 638).

05.Subject Detection in Each Mode

Subject detection is the camera recognising people, animals, birds, vehicles or aircraft and steering focus to an eye or face where it finds one, and otherwise to the subject itself, such as a car or a plane (pp. 127-128). Nikon lists it during autofocus for Wide-area AF (S), (L), (C1) and (C2), 3D-tracking, Subject-tracking AF and Auto-area AF (p. 128). Pinpoint AF, Single-point AF and Dynamic-area AF are absent from that list, so there the camera focuses on your point, or in Dynamic-area AF on its neighbours when the subject briefly leaves it, whatever subject class is set.

Auto-area AF searches the whole frame. For manual focus, Nikon says the detection options Wide (L) and Wide (S) confine detection to the current focus area (p. 128), but it does not say the same of the autofocus boxes. Expect a bird outside a small box to be ignored, and confirm it in your own viewfinder. Nikon's one concrete tip is that if Birds keeps locking onto things that are not birds, a mode with a smaller area may help (p. 129). Subject Detection covers the classes and their failure cases.

06.Recommended Settings

These are my choices for the shooting this guide follows. If the camera does not offer a box size exactly, take the nearest.

Birds in flight. On the 180-600 my choice is Wide-area AF (L) with Birds detection. Against open sky nothing nearer competes for focus, and detection finds the head for you. It is also the wide preset I hold on the sub-selector centre for take-off. Once the bird is steady in the frame, 3D-tracking is the alternative, following it from the point where I started (p. 125). Low over reeds or in front of trees, the nearest-subject rule can still hand focus to a stem (p. 124). There I use Dynamic-area AF (L), Nikon's own example for fast birds: it focuses on my point and turns to the neighbours only when the bird leaves it (p. 124), at the cost of subject detection (p. 128).

Perched wildlife. A $3 \times 3$ box in C1 with Birds keeps detection with only nine points for a twig to hide in, against 81 for $9 \times 9$. Behind a screen of branches, Single-point AF threads one gap. I leave Pinpoint AF out for wildlife, since it needs AF-S and focuses more slowly (p. 123), and a resting bird can hop at any moment.

Stage dance. On the 70-180 I set C1 to a tall, narrow box shaped to one dancer, with People. At 15 m and 180 mm a 1.7 m dancer is $1.7 \times 180 / 15 \approx 20$ mm tall on the 24 mm-high sensor, about 11 of the 13 rows, and her half-metre width spans three or four of the 21 columns. She is nearer than every piece of scenery, so the nearest-subject rule works for her, and the narrow box leaves little room for a second body crossing downstage. I avoid 3D-tracking on stage, because colour and brightness changes are on Nikon's list of what defeats tracking (p. 126), and stage lighting changes both constantly. Outdoors, where daylight does not shift colour that way, 3D-tracking with People is my choice.

Portraits. On the Nikkor Z 85mm f/1.8 S at 2 m and f/1.8, $2 \times 1.8 \times 0.00003 \times 2^2 / 0.085^2 \approx 0.06$ m, about 6 cm of depth. That is less than the depth of a face from nose tip to ears, so the camera must choose the eye. Auto-area AF with People does that, Nikon recommends it for portraits (p. 125), and left and right switch faces in a group (p. 128).

Street and landscape. Street rarely leaves time to place a point, so Auto-area AF with People is the honest choice. A landscape is the opposite: nothing moves, so Single-point AF or Pinpoint AF in AF-S puts focus exactly where you decide.

Video on a gimbal. Following a dancer, I use Wide-area AF (L) with People and Full-time AF. Nikon suggests the wide-area modes for smooth focus in moving video shots (p. 124), and I take the larger. Subject-tracking AF is the alternative for a single performer.

07.Beginner Mistakes

Leaving Single-point AF on for birds in flight. It is the default (p. 447), and one point cannot follow a bird your pan does not hold perfectly. Change modes before the bird moves.

Trusting a wide area in front of clutter. The nearest subject wins (p. 124). Twigs, a downstage dancer and the front row are all nearer than what you came for. Shrink the area, or use a mode where your point has priority.

Expecting subject detection everywhere. It is not offered in Pinpoint AF, Single-point AF or Dynamic-area AF (p. 128). If the eye box never appears, check the area mode first.

08.Related Chapters

• Focus Modes: AF-S, AF-C and AF-F, and which area modes each allows.
• Subject Detection: the subject classes, eye detection and when detection fails.
• The Focus Custom Settings: a4, a7, a8 and a10 with the rest of the a group.
• Back-Button Focus: autofocus on the AF-ON button, and the Pinpoint AF caution.
• How Autofocus Works on the Sensor: phase and contrast detection from first principles.
• Choosing an Autofocus Setup: complete setups combining area mode, focus mode and controls.
• Menus and Controls: the sub-selector centre and Fn2 assignments used above.
• Metering: spot metering, which follows the focus point.