Back-Button Focus

A kingfisher drops onto a bare branch above a stream, and you have it at 600 mm on the Nikkor Z 180-600mm f/5.6-6.3 VR. It may sit for ten seconds or for five minutes, and when it leaves it will be gone faster than you can react. With the camera set as it comes from the box, one finger does two jobs. A light press of the shutter-release button focuses, and pressing it the rest of the way takes the picture. Keeping the bird sharp through a long wait means holding that light press the whole time, and each time you relax the finger, the next press starts focusing again from nothing.

Back-button focus gives the two jobs to two fingers. Your right thumb presses AF-ON, a button on the camera's rear, to focus, and your index finger presses the shutter-release button only to take pictures. Thumb down, the camera focuses. Thumb up, focus stays where it was. The index finger fires whenever you choose and never moves the focus. One Custom Setting, a6 AF activation, makes the change, and this chapter follows what it does to everything that used to hang on the light press. Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.Two Jobs on One Button

What the Half-Press Does by Default

The shutter-release button has two stops. Pressed lightly, it halts at a point of gentle resistance, which Nikon calls pressing it halfway and this guide calls a half-press. Pressed through that point, it takes the picture. Autofocus, the camera driving the lens until the subject is sharp, is normally started by the half-press. The default for a6 is Shutter/AF-ON (p. 623), under which a half-press starts autofocus and so does the AF-ON button (pp. 135, 637).

What happens next depends on the focus mode. In AF-S, Nikon's Single AF, the camera focuses once and freezes there for as long as the half-press lasts (pp. 121, 135). In AF-C, Continuous AF, it keeps adjusting while the button stays half-down, following the subject as it comes closer or moves away (p. 121). The Focus Modes chapter treats both in depth. In either mode, the finger that decides when a picture is taken also decides what focus is doing, so you cannot change one without disturbing the other.

The Perched Bird

Back to the kingfisher. In AF-C you want the camera tracking when it leaves, because its distance from you changes between one frame and the next. While it sits, you want focus left alone, because the camera has nothing useful to do and something unhelpful it might do. A reed swaying through the focus point is a change of distance, and AF-C responds to those. Holding a half-press for minutes also tires the finger that must move fastest when the moment comes.

With back-button focus, you press the thumb as the bird lands, let the camera find it, and lift. Focus now stays at the branch (p. 135). You can take frames of the perched bird with the index finger, and none of them moves the lens. When the bird crouches to leave, the thumb goes down, AF-C starts following, and the index finger holds the burst.

The Recomposed Portrait

With the Nikkor Z 85mm f/1.8 S wide open at f/1.8, the zone of sharpness in a head-and-shoulders portrait is shallow enough that focus has to land on the eye. Suppose the picture you want puts the face near the edge of the frame, away from the focus point. The usual answer is to focus on the eye, then move the camera while keeping focus frozen. In AF-S, the half-press itself holds focus while you recompose (p. 135). In AF-C a half-press does not hold focus, so Nikon's route is to half-press and then press the centre of the sub-selector, the small joystick beside the AF-ON button, which holds focus and exposure for as long as you keep it pressed (pp. 134, 914).

Back-button focus folds that into one gesture. Focus with the thumb, lift it, recompose, and take as many frames as the moment allows. The one rule that survives is distance. If you or the sitter move closer or further after focusing, the frozen focus is wrong, and Nikon's advice for focus lock applies here unchanged: focus again at the new distance (p. 134). Subject detection, which finds the eye for you, often removes the need to recompose, as Subject Detection explains.

Tracking and Holding Without a Mode Change

The two scenes share one idea. With back-button focus in AF-C, you have continuous tracking while the thumb is down and a frozen focus while it is up, with no menu, no button combination and no change of focus mode. The thumb becomes the mode switch.

A dancer performing outdoors shows why that matters. She crosses the grass and you track her with the thumb down. She stops in a held balance for two seconds, and you lift the thumb so the camera stays on her rather than drifting to the dancer behind. She moves again and the thumb goes back down. Doing the same with the focus mode itself means the i menu or the Fn2 button, which by default carries focus mode and AF-area mode, the setting that decides which part of the frame the camera focuses on (pp. 121, 625). Nobody does that three times in ten seconds.

02.Custom Setting a6 AF Activation

AF Activation

a6 decides whether the shutter-release button may start autofocus (p. 637). It has two options, and the second carries a sub-option that also sets how a full press behaves.

Shutter/AF-ON is the default (p. 623). A half-press focuses, the AF-ON button focuses, and either will do.

AF-ON only takes focusing away from the half-press, and a full press still takes pictures, subject to the sub-option below. Focus starts from the AF-ON button or another control assigned AF-ON (p. 637). This option is back-button focus.

To reach the sub-option, highlight AF-ON only and press right on the multi selector, the ring on the rear you press in four directions, and Out-of-focus release appears with two choices (p. 637). Two terms first. Release priority means the shutter fires the moment you press the button fully, sharp or not. Focus priority means the camera holds the shutter back until it judges the subject to be in focus.

Enable gives release priority, so a full press takes a picture whether or not the subject is sharp (p. 637).

Disable gives focus priority, so a full press takes a picture only once the camera has focus (p. 637). Nikon's online manual puts its reminder to focus with AF-ON in this same row, and the pairing makes sense: with Disable, the thumb has to bring the subject into focus before the index finger can take anything.

The Reference Guide prints no default for Out-of-focus release. Its list of Custom Settings defaults gives a6 as Shutter/AF-ON and stops there (p. 623), so look at which choice your camera shows the first time you open the sub-option rather than assuming one.

Does a6 change anything else about the half-press? The Reference Guide describes it only in terms of focus. Other features hang on the half-press too: with c1 Shutter-release button AE-L at On (half press) it locks exposure (p. 659), and Pre-Release Capture buffers during it (p. 665). Neither page mentions a6. My reading is that AF-ON only removes focusing from the half-press and leaves the rest alone. That is an inference from what Nikon does not say, and the Pre-Release Capture section below gives a way to check it on your own camera.

03.The AF-ON Button

It sits at the top right of the camera's rear, under the right thumb (p. 50). Nikon's own account of it is short (p. 135). It can start autofocus in place of the shutter-release button. Under AF-ON only, it is how focusing begins and a half-press no longer does anything to focus. When you let go of it, focus stays frozen, and pressing it again starts a fresh focus.

That last point matters in AF-S. Since each new press of AF-ON is a new focus, tapping the thumb twice out of habit throws away a focus you meant to keep. In AF-C the same press simply resumes tracking. It is also the cure when the focus point flashes while AF-ON is held, Nikon's sign that the camera has lost the ability to focus: lift and press again (p. 913).

The button's role is itself a setting. In f2 the AF-ON button entry defaults to AF-ON (p. 625), and it can be handed other jobs. If you ever do that while a6 is on AF-ON only, the camera can focus only through whatever controls still carry AF-ON or AF-area mode + AF-ON, which also starts autofocus (p. 705), and that may be none.

Other Controls That Can Focus

Custom Setting f2 Custom controls (shooting) gives roles to 22 entries in photo mode, from the Fn buttons to the lens rings and the command dials, and Nikon notes that the roles on offer differ from control to control (pp. 703-704). Five of the roles start or hold autofocus.

• AF-ON starts autofocus, doing the same job as the AF-ON button (p. 705).
• AF-area mode + AF-ON switches to an AF-area mode chosen in advance and starts autofocus while held, then returns to the previous mode on release (p. 705).
• AF lock only holds focus while pressed (p. 706).
• AE/AF lock holds focus and exposure while pressed (p. 706).
• Same as AF-ON button makes a control copy whatever the AF-ON button currently does (p. 712).

Three controls already focus out of the box (pp. 625-626). They are the AF-ON button itself on AF-ON, the Lens Fn2 button on AF-ON for lenses that have one, and the AF-ON button for vertical shooting on an optional battery pack, set to Same as AF-ON button. The Sub-selector center, the Lens Fn button and the Vertical multi selector center default to AE/AF lock (p. 626). The Fn button of an optional wireless remote controller can take AF-ON too (p. 853). Deciding which control does what belongs to Custom Controls.

Video has its own control map in Custom Setting g2 (p. 748). It adds Fast AF-ON, which focuses at full speed in AF-C whatever g5 AF speed says (p. 750). That belongs to the video part of this guide.

04.How It Meets the Rest of Autofocus

AF-S and AF-C

In AF-S, back-button focus changes little. The thumb focuses once, and focus stays frozen whether the thumb stays down or lifts (pp. 121, 135). It is a tidier way to focus and recompose.

In AF-C it turns constant tracking into tracking on demand, with a frozen focus between presses, which covers most of what AF-S was for. Once focus is on the thumb I leave the camera in AF-C for anything that might move. AF-S keeps one thing AF-C lacks at default settings, focus priority, which the next section explains.

Release Priority and Focus Priority

Two Custom Settings already choose between release priority and focus priority, one for each mode. a1 AF-C priority selection offers Release, Focus + release and Focus, with Release the default (pp. 623, 631). Focus + release gives focus priority to the first frame of a burst when the subject is dark or low in contrast, and release priority to the frames after it (p. 631). a2 AF-S priority selection offers Release and Focus, with Focus the default (pp. 623, 632).

Out-of-focus release asks the same question a third time, and the Reference Guide never says which answer wins when a1 or a2 disagrees with it. My inference is that under AF-ON only the sub-option governs the full press, since it exists only there and is defined in the same two terms. The practical answer is to make them agree so the question never comes up. That means Enable with Release, or Disable with Focus. If a camera set up this way ever refuses to fire, check all three settings before blaming the subject.

Subject Detection and AF-Area Modes

Subject detection is the camera recognising a class of subject, such as people, animals, birds or vehicles, and steering focus to its face or eye, or for vehicles and aircraft to the machine itself (pp. 127-128). It works only in certain AF-area modes: the four wide-area modes, 3D-tracking, Subject-tracking AF in video, and Auto-area AF (p. 128). Nothing in the Reference Guide makes detection depend on which button starts autofocus. Since the AF-ON role does what the AF-ON button does (p. 705), and a6 only decides whether the half-press may also start it (p. 637), I expect detection to behave the same under the thumb. That is an inference, not a statement from Nikon. The AF-Area Modes chapter describes each mode.

3D-tracking, which hands a subject from one focus point to the next as it moves across the frame, deserves a note. AF-ON or a half-press starts it, and letting go ends tracking and puts the focus point back where you had placed it (p. 125). With focus on the thumb, lifting the thumb therefore ends tracking and returns the point to your starting position. Reading pp. 125 and 135 together, I take it that the lens stays where tracking left it.

The AF-area mode + AF-ON role turns a second button into a second camera (p. 705). Keep Wide-area AF (C1) set small for the AF-ON button, for a bird among branches. Give the sub-selector centre AF-area mode + AF-ON, if the camera offers that role there, with Wide-area AF (L) as its preset, for a bird against open sky. Detection is available in both modes, with Birds chosen in AF/MF subject detection options (pp. 127-128). Each button then picks its area and starts focusing in one press. My own U1 does the same on the half-press instead, as the costs section explains.

There is a trade-off. Custom Setting a7 Focus point persistence decides which point survives when a button like that takes you from a mode in which the camera picks the point, such as Auto-area AF, to one in which you pick it, but Nikon says it applies only while you are focusing with a half-press (p. 638). Under AF-ON only that condition never arises, so I expect a7 to have nothing to act on. The Focus Custom Settings covers a7 and the rest of the a group.

Pre-Release Capture and the Half-Press

Pre-Release Capture is the feature most bound to the half-press, and the Reference Guide is exact about its trigger. It works in the high-speed frame capture + release modes, C15, C30, C60 and C120, which this guide shortens to HSFC+ (p. 156). When Custom Setting d3 Pre-Release Capture options has Pre-release burst set to anything other than None, the camera fills its buffer, the fast temporary memory that holds frames before they reach the card, for as long as the shutter-release button is half-pressed. A full press then writes the last stretch of them to the card (p. 665). Every time, Nikon names the shutter-release button as the trigger and never AF-ON, so do not expect the thumb to start buffering.

A green dot in the Pre-Release Capture icon shows that buffering is under way (p. 665). After about 90 seconds of unbroken half-press the feature cancels itself, with the icon flashing for the 30 seconds before, and the cure is to lift the finger and half-press again (p. 665).

Under AF-ON only, the two fingers separate further. Because Nikon describes the buffering without reference to a6, I expect a half-press to keep buffering even though it no longer focuses. Watch for the green dot the first time you try it, since that is the camera's own answer. If the dot appears, the perched kingfisher becomes an even better case. The thumb sets focus and lifts, the index finger half-presses to buffer, and each lift and re-press to beat the 90-second limit leaves focus untouched, because under AF-ON only a half-press does not focus (pp. 637, 665). Re-pressing when the icon starts to flash, about 60 seconds in, as Pre-Release Capture advises, splits a five-minute wait into $300 / 60 = 5$ stretches, so four re-presses, each of which would also restart focusing under the default.

Ordinary bursts, covered in Release Modes, behave the same way. Under AF-ON only a full press fires them and focus follows the thumb alone, so a burst in AF-C with the thumb up is shot entirely at the frozen distance. To track during a burst, keep the thumb down while the index finger holds the shutter-release button.

05.What It Costs

The first cost is a habit. Years of half-pressing do not vanish in a week, and the early failures are frames taken at the focus of a minute ago because the thumb never went down. With a long lens, holding thumb and index finger down together for seconds at a time also takes practice.

The second cost falls on whoever else picks up the camera. A borrower half-presses and nothing focuses. With Enable, they get pictures focused wherever the lens last stopped. With Disable, the shutter may not fire at all, which looks like a fault.

There are three remedies, from least effort to most. Tell the borrower to use the thumb. Set a6 back to Shutter/AF-ON before handing the camera over. Or keep a user setting position on the mode dial with the default a6 for other people, since Custom Settings are among the things U1 to U3 can store (p. 142). Reset custom settings also brings back Shutter/AF-ON (pp. 623, 630), but it resets every other Custom Setting with it, so keep it for a clean start rather than a quick handover.

The third cost is anything you have built on the half-press. a7, discussed above, is one. The bird-in-flight arrangement in Menus and Controls is another: it holds a wide AF-area mode on the sub-selector centre with plain AF-area mode while the half-press focuses, which is why my U1 for the 180-600 keeps the a6 default.

06.Recommended Settings

For a perched bird with the 180-600, I use the table as it stands, with Wide-area AF (C1) set small and Birds detection. The thumb holds the sitting bird at a fixed focus while you wait, and goes down the moment it moves. I choose Enable over Disable here because Nikon does not say whether Disable lets a frame fire once the thumb has lifted to hold focus (p. 637), and holding focus is the whole point. If the sub-selector centre is not doing AF-area work, give it AE lock only, as the Metering chapter suggests, so freezing exposure never freezes focus. For birds in flight I keep the half-press on U1, for the reasons given under costs.

Dance in daylight takes the same settings. Stage dance with the Tamron 70-180mm f/2.8 Di III VC VXD G2 is the one place I leave a6 at Shutter/AF-ON. On a dim stage I want a1 at Focus + release, so the first frame of a burst on a dark dancer waits for focus (p. 631), and the Reference Guide never describes that choice under AF-ON only. Choosing an AF Setup gives the full setup for each.

For portraits with the 85mm I stay in AF-C, with Auto-area AF and People detection, a6 at AF-ON only, Disable and a1 at Focus, so both settings ask for a sharp frame. A sitter is still enough that the wait costs little, at the price of the borrower problem described under costs. Landscapes in AF-S take Disable with a2 at Focus, remembering the Pinpoint AF exception above.

07.Beginner Mistakes

Half-pressing out of habit. Under AF-ON only nothing focuses, and the picture lands wherever the lens last stopped. Thumb first, always.

Choosing Disable and thinking the camera is broken. With focus priority, a full press does nothing until the thumb has brought the subject into focus (p. 637). Use Enable for fast or unpredictable subjects.

Expecting the thumb to start Pre-Release Capture. Nikon names only the shutter-release button's half-press as the buffering trigger (p. 665).

Lending the camera without a word. The borrower gets either soft pictures or a shutter that will not fire. Reset a6 or use a spare user setting first (p. 142).

08.Related Chapters

• Focus Modes: AF-S and AF-C, and how each behaves on a half-press or a thumb.
• The Focus Custom Settings: a1, a2 and a7, the settings back-button focus leans on or switches off.
• AF-Area Modes: the areas a second button can switch to, and 3D-tracking.
• Subject Detection: finding an eye in a wide area, which often replaces recomposing.
• Pre-Release Capture: the buffer that fills during a half-press.
• Release Modes: single frames, bursts and the HSFC+ modes the thumb now focuses for.
• Menus and Controls: reaching the custom settings menu, and the user setting positions that hold a6.