Focus Modes

An egret lifts off a sandbar 30 metres away and flies straight at you. You have it at 600 mm on the Nikkor Z 180-600mm f/5.6-6.3 VR, the shutter is open for a two-thousandth of a second, and the frame comes back soft. Nothing was wrong with the focusing. The camera focused precisely on the bird, at a distance the bird had already left.

Whether the camera focuses once and stops, or keeps measuring and aims at where the subject is going, is the focus mode. It is one setting, with three choices for stills and four for video, and the arithmetic that shows why it matters fits in a few lines. Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.Focusing Once or Following

What the Lens Is Doing

A lens bends the light from each point of the subject so that it converges again behind the glass. Where it converges depends on distance: light from a bird 30 m away comes together slightly farther behind the lens than light from a hill three kilometres off. Focusing slides glass elements inside the lens until the convergence point for the distance you care about lands on the sensor. Points nearer or farther land slightly in front of or behind the sensor and record as small discs instead of points.

A disc small enough still looks like a point. The largest disc that passes for a point is the circle of confusion, 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. Autofocus, shortened to AF, is the camera moving those elements for you. How the Z6III judges sharpness with detectors built into the sensor is the subject of How Autofocus Works on the Sensor. This chapter is about when the camera measures, and for how long.

Servo: Measure, Correct, Repeat

Nikon's specifications call the focusing system a lens servo (p. 1002). A servo is any control loop that measures an error, drives a motor to shrink it and measures again, the way a thermostat keeps checking a room. For focus, the error is the gap between the distance the lens is set to and the distance of the subject. The focus modes differ in one thing: how long that loop is allowed to run.

Single AF: Measure Once and Hold

Single AF, shown as AF-S and called single-servo AF in the specifications (p. 1002), runs the loop on a half-press, which means pushing the shutter-release button down to its first resistance point without taking the picture, and stops it as soon as the error is near zero. The focus point turns from red to green, and the lens stays where it is for as long as you keep the button half-pressed (pp. 121, 135).

Stopping is the point. Once focus is locked you can move the camera to put the subject off-centre and shoot, and the lens will not wander off to whatever now sits under the focus point. Nikon recommends AF-S for subjects that are not moving (p. 121): a building, a landscape, a product on a table. Its weakness follows from the same fact. If the subject changes distance after the lock, the camera does not know.

Continuous AF: Keep Measuring

Continuous AF, shown as AF-C and called continuous-servo AF in the specifications (p. 1002), never stops the loop while the half-press lasts. The camera keeps adjusting focus as the distance to the subject changes, and Nikon's own example is athletes (p. 121). Nothing is locked by the half-press, so if you recompose, the camera refocuses on whatever the focus point now covers.

Why Following Is Not Enough

Measuring continuously still leaves a gap. Between the camera's last distance reading and the moment the exposure starts, the camera has to compute, move the focusing elements and open the shutter. Nikon publishes no figure for that interval, so take 50 ms, fifty thousandths of a second, as an illustrative value. It is convenient rather than measured: at 20 frames per second (fps), the top rate of Continuous H (extended) with the electronic shutter (pp. 153-154, 1000), frames also arrive $1/20$ s, or 50 ms, apart.

Now give the egret an illustrative speed of 15 metres per second, a round number for the arithmetic rather than a measured flight speed. In 50 ms it closes

$$\Delta d = v\,\Delta t = 15 \times 0.050 = 0.75\ \text{m}.$$

Here $v$ is the closing speed and $\Delta t$ the lag. Focus set at the 30.00 m the camera measured is aimed at a bird now at 29.25 m. Whether that matters depends on the depth of field. With $f$ the focal length, $N$ the f-number, $s$ the focus distance and $c$ the circle of confusion, the total depth $D$, near side plus far side, is approximately

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

The approximation holds while $s$ is far short of the hyperfocal distance, the nearest focus distance whose sharp zone reaches to infinity, about $f^2/(Nc)$, which is roughly 1.9 km here. For $c$ take 0.03 mm, a common convention for a 36 by 24 mm frame and not a Nikon figure. At 600 mm and f/6.3, with every length in metres:

$$D \approx \frac{2 \times 6.3 \times 0.00003 \times 30^2}{0.6^2} = \frac{0.340}{0.36} \approx 0.95\ \text{m}.$$

That is about 0.47 m either side of the focus distance, and the standard near-limit formula puts the near edge at 29.53 m. The bird, at 29.25 m, is 0.28 m nearer than the sharp zone reaches. The miss is $0.75/0.47 \approx 1.6$ times the near half of the depth of field.

Closer in, the picture gets worse. Depth of field grows with the square of distance, so at 15 m it is a quarter as deep, about 0.24 m, or 0.12 m each side. The bird still covers 0.75 m in 50 ms, now more than six times the half-depth. A closing subject outruns the sharp zone fastest at exactly the distances where it fills the frame.

The cure is to aim at where the bird will be. The camera cannot measure speed directly, but it can infer it from two readings. Suppose it read 30.30 m and then 30.00 m, 20 ms apart. With $d_0$ and $d_1$ the two readings and $\tau$ the time between them, the closing speed is $v = (d_0 - d_1)/\tau = 0.30/0.020 = 15$ m/s, and the forecast distance after a further $\Delta t$ is

$$\hat{d} = d_1 - v\,\Delta t = 30.00 - 15 \times 0.050 = 29.25\ \text{m}.$$

The hat on $\hat{d}$ marks it as an estimate. Focus set there puts the bird in the middle of a sharp zone running from 28.81 to 29.71 m, as the lower row of the figure shows.

The forecast assumes the bird holds its speed. If it starts braking just after the last reading, at a steady $a$ metres per second per second, the forecast misses by $\tfrac{1}{2} a\,\Delta t^2$. At an illustrative $20\ \text{m/s}^2$, hard enough to stop from 15 m/s in three quarters of a second, the miss is $0.5 \times 20 \times 0.05^2 = 0.025$ m, 2.5 cm, well inside a 47 cm half-depth. Ignoring speed costs an error that grows with $\Delta t$, while ignoring a change of speed costs one that grows with $\Delta t^2$, and over a few hundredths of a second a square is small. So the forecast does not need to be perfect, only roughly right.

This is the principle, worked with numbers of my own. The Z6III Reference Guide lists predictive focus tracking among the focusing features (p. 1002) but says nothing of how it works or which focus modes use it. Nikon's D850 manual ties it to AF-C, and I assume the Z6III does the same. AF-S stops measuring once it locks, so it has nothing to extrapolate from, and whether AF-F predicts in video is my guess. Prediction is the practical reason to use AF-C for anything that approaches or recedes.

Full-Time AF and Manual Focus

Full-time AF, shown as AF-F, exists only in video mode (pp. 121, 1002). The loop runs with no button held at all, following the subject and any change of framing, and a half-press stops the loop instead of starting it: the point turns green and focus locks until you let go (p. 121). That suits a camera on a gimbal, where your hands are busy steering.

Manual focus, shown as MF, hands the loop to you. You turn the focus ring or the control ring, and the camera still measures: a focus indicator shows whether the subject under the point is in focus or whether focus lies in front of or behind it, and the point turns green when it is sharp (pp. 136-137). Nikon calls this electronic rangefinding. The shutter fires whether or not the subject is sharp (p. 121). Magnification, focus peaking and the other aids belong to Focus Limiter, Saved Positions and Manual Focus.

02.The Focus Mode Items

Photo and Video Are Set Separately

The stills item offers Single AF, Continuous AF and Manual focus, since Full-time AF is video only (p. 121). It starts at Single AF (p. 447).

The video item adds Full-time AF to those three (p. 121) and starts at Full-time AF (p. 579). Its entry (p. 604) points back to the focus mode descriptions on p. 121 and mentions no option that copies the photo setting, and the two items start at different defaults, so I read them as holding separate values. Switching stills to AF-C should leave video where it was, but check before recording. Video autofocus speed and tracking belong to the video part of this guide.

Changing the Focus Mode

The menus are the slow route. Focus mode sits in both default i menus, the quick-settings screens the i button opens (pp. 83-84, 625, 627). Faster still, the Fn2 button's default role in photo and video is Focus mode/AF-area mode (pp. 74, 625, 627): hold Fn2 and turn the main command dial to change the focus mode, or the sub-command dial to change the AF-area mode (pp. 713, 755). If you would rather have those two dials the other way round, the command-dial role Focus/AF-area mode selection in Custom Setting f2 swaps them for photos (p. 716), and g2 does the same for video (p. 757). On a lens with its own focus-mode switch, that switch selects manual focus (p. 137).

Moving Fn2's role to another button belongs to Custom Controls. I leave it where it is. One hold-and-turn takes AF-S to AF-C with the camera at your eye.

The Shutter-Release Button in Each Mode

In AF-S, a half-press focuses and holds. At default settings, if the camera cannot find focus, the point flashes red and the shutter will not fire (p. 121). Keep the half-press between frames and every frame uses the same distance, and pressing the centre of the sub-selector, the small joystick on the back, locks focus too (p. 135). At default settings, the self-timer will not start if AF-S cannot focus (p. 158).

In AF-C, a half-press keeps focusing and never locks. At default settings the shutter fires whether or not the subject is sharp (p. 121). To freeze focus while you recompose, hold the half-press and press the centre of the sub-selector, which locks exposure as well (pp. 134, 914). The point does not turn green by default: Custom Setting a10's AF-C in-focus display starts at OFF (p. 623), and the point then stays red or yellow whether or not the camera has focus (p. 642). If the point flashes during a half-press, the camera can no longer focus, and letting go and pressing again restarts the search (p. 913).

In AF-F, the camera focuses with no press, and a half-press locks (p. 121). In MF, the button only takes pictures, sharp or not (p. 121).

The AF-ON button under your thumb can do the focusing in place of the half-press (p. 135). Choose AF-ON only for Custom Setting a6, AF activation, and the shutter-release button stops focusing altogether (p. 637), and focus stays where it was when you lift your thumb off AF-ON (p. 135). That is Back-Button Focus, and it changes what AF-C means: hold the thumb down and the camera follows, lift it and the camera holds. One mode then does both jobs.

Release Priority and Focus Priority

Every autofocus camera has to settle one question: when you press the button fully and the camera has not confirmed focus, should it fire anyway? Focus priority makes it wait until it judges the subject sharp. Release priority makes it fire the instant you press. Focus priority trades a missed moment for fewer soft frames, and release priority trades the other way.

The defaults split along the modes. In AF-S the camera waits, because Custom Setting a2, AF-S priority selection, starts at Focus, and the alternative is Release (pp. 623, 632). In AF-C it fires, because a1, AF-C priority selection, starts at Release, with Focus + release and Focus as the alternatives (pp. 623, 631). The logic holds up. A subject you chose AF-S for is assumed still, so waiting a moment costs little. A subject you chose AF-C for is moving, and the moment will not come back. A portrait sitter is my exception, where a1 at Focus costs little. What each option does in detail, including the first-frame rule in Focus + release, is covered in The Focus Custom Settings.

One exception, and one setting to keep in step. When Interval priority is ON for interval-timer photography or a time-lapse video, the camera uses release priority whatever a1 and a2 say (pp. 523, 533). And with a6 at AF-ON only, the a6 item adds its own Out-of-focus release setting, where Enable means release priority and Disable means focus priority (p. 637). The Reference Guide does not say which wins when it disagrees with a1 or a2, so I set them to agree, as the back-button chapter explains.

Where the Camera Decides for You

Some AF-area modes, the settings for where in the frame the camera looks, are tied to one focus mode. In photo mode, Pinpoint AF needs Single AF, while the three Dynamic-area AF sizes and 3D-tracking need Continuous AF (pp. 123-125). If a mode you want is missing, check the focus mode first. AF-Area Modes covers each one.

Several functions set the focus mode themselves. Pixel shift shooting fixes it at AF-S and drops an AF-C-only area mode back to Single-point AF (pp. 548, 558). Auto capture switches to AF-C for as long as it runs, if autofocus is on (p. 562). Focus shift needs AF-S or AF-C (p. 542). The built-in AF-assist illuminator, a lamp on the front that lights a dim subject at about 1 to 3 m, lights only in AF-S in photo mode, never in AF-C or MF (pp. 643, 916).

03.Recommended Settings

Wildlife and birds. With the 180-600mm I use AF-C for everything, perched birds included. A perched bird preens, leans and turns its head, and a heron that has stood still for ten minutes can leave without warning. With back-button focus, a still subject costs nothing in AF-C. For flight, AF-C is the only stills mode in which the camera keeps measuring and moving the lens, and so the only one in which the prediction above can work. I leave a1 at Release, perched or flying, because a slightly soft frame in a burst is worth more than no frame at all.

Stage dance. With the Tamron 70-180mm f/2.8 Di III VC VXD G2, AF-C. A dancer crossing the stage changes distance continuously, and at f/2.8 the sharp zone is shallow. The AF-assist lamp does not light in AF-C (p. 916), and a stage is well beyond its reach of about 3 m anyway (p. 643).

Portraits. With the Nikkor Z 85mm f/1.8 S wide open at 2 m, the same formula gives a sharp zone about 6 cm deep:

$$D \approx \frac{2 \times 1.8 \times 0.00003 \times 2^2}{0.085^2} = \frac{0.000432}{0.007225} \approx 0.060\ \text{m}.$$

That is 3 cm either side, and a sitter who leans in to laugh moves farther than that. AF-S locks on the eye and the eye leaves, so I use AF-C, and keep AF-S for a sitter who is genuinely still. Subject detection then keeps the point on the eye, which Subject Detection covers.

Street. AF-C, because the people you photograph are walking, often straight at you. Manual focus set in advance to a distance, called zone focusing, is the classic alternative, but it needs a deep sharp zone, which means a short focal length and a small aperture.

Landscape. AF-S on a tripod. The scene does not move, locking once is what you want, and AF-S is the only mode that offers Pinpoint AF (p. 123). For a night sky or a low-contrast scene that defeats autofocus, switch to MF and zoom in on the view with the X button (pp. 122, 136).

Gimbal dance video. Leave the video item at its default, Full-time AF. Your hands are on the gimbal, not on the shutter button, and AF-F keeps focusing with nothing held. The video part of this guide covers AF speed and tracking sensitivity.

04.Beginner Mistakes

Using AF-S for a moving subject. AF-S locks a distance, and the subject leaves it. If anything in the frame approaches or recedes, use AF-C.

Expecting a half-press to lock in AF-C. It never does. Press the centre of the sub-selector while half-pressing (p. 134), or use back-button focus and lift your thumb.

Reading the AF-C focus point as a promise. By default it stays red or yellow whether or not the camera has focus (pp. 623, 642), and the shutter fires either way (p. 121). If you skipped first-day setup, turn on AF-C in-focus display in a10 for a green confirmation.

Leaving the lens switch on manual. If every frame is soft and the camera seems to ignore the half-press, check that it is not in manual focus, from the menu or from a switch on the lens (pp. 137, 913).

Hunting for Dynamic-area AF under AF-S. It and 3D-tracking appear only with Continuous AF, and Pinpoint only with Single AF (pp. 123-125).

Assuming video follows stills. The two focus mode items are separate, starting at Single AF and Full-time AF (pp. 447, 579). Check the video item before you press record.

05.Related Chapters

• How Autofocus Works on the Sensor: how the sensor measures focus, the measurement each focus mode repeats or stops.
• AF-Area Modes: where in the frame the camera looks, and which modes need AF-S or AF-C.
• Back-Button Focus: moving focus to the AF-ON button, which makes AF-C do the work of AF-S.
• The Focus Custom Settings: a1 and a2 priority selection and the rest of the a group.
• Subject Detection: keeping the focus point on an eye, an animal or a vehicle.
• Focus Limiter, Saved Positions and Manual Focus: manual focus aids and limiting the distance the lens searches.
• Release Modes: burst rates, including the 20 fps used in the prediction example.
• Choosing an Autofocus Setup: complete autofocus setups for each kind of shooting.