Continuous Autofocus (AF-C): How It Works and When to Use It

Continuous autofocus (AF-C on most cameras, AI Servo or Servo AF on others) keeps adjusting focus for as long as you half-press the shutter button or hold the AF-ON button. It never locks. The camera measures the subject’s distance again and again, works out how fast it is approaching or receding, and sets the lens for where the subject will be at the instant of exposure. Use it for anything that changes distance from you.

That last idea is the whole subject. What matters is not how fast something moves, but how fast its distance from you changes compared with the depth of the sharp zone. This page shows that with numbers, then covers how the camera keeps up, the settings that tune it, the AF area to pair with it, and when it is the wrong tool. The three focus modes are introduced side by side in autofocus modes explained.

What continuous autofocus does while you hold the button

In single-shot AF the camera focuses once, confirms, and stops. If the subject then moves half a meter, the lens stays where it was. In continuous AF the camera repeats the measurement many times a second and moves the lens after each one, so focus follows the subject for as long as the button is held.

Following is only half the job. There is always a short delay between the last focus measurement and the exposure: the lens has to move and the shutter has to open, and on a camera with a mirror the mirror lifts and blinds the focus sensor. A mirrorless camera reads focus from the image sensor between frames, so the gap is different but still there. The subject keeps moving during it. So the camera uses its recent measurements to estimate the subject’s speed along the lens axis, then drives the lens to the distance the subject should have reached when the exposure starts. This is predictive autofocus, and it is the reason AF-C can keep an approaching subject sharp.

• Speed toward or away from you is what counts. A subject crossing the frame at a steady distance barely changes focus, however fast it goes. One coming straight at the camera changes it fastest. Most real subjects do some of each, and at close range even a crossing subject changes distance enough to matter.
• Prediction needs a history. The camera cannot estimate speed from one reading. Start focusing a second or two before the moment you want and keep the button held. Letting go and pressing again makes the camera start its calculation over.
• Focus is not freezing. Continuous AF puts the subject at the right distance. It does nothing about motion blur, which is the job of the shutter speed. Action needs both.

What is happening inside the camera when it measures distance? How autofocus works covers the two fundamental approaches to autofocus, hybrid systems, and eye AF and subject detection.

The focus budget: how long a moving subject stays sharp

Here is why prediction exists. On a full frame camera, a 200mm lens at f/2.8 focused at 10 meters has about 41 cm of depth of field, roughly 20 cm in front of the focus distance and 21 cm behind it. A sprinter or a cyclist at a brisk pace covers about 10 meters a second. Coming straight at you, that subject crosses the whole sharp zone in about 41 milliseconds. Focus that was right a twentieth of a second ago is already wrong.

The table runs the same calculation for other cases: the depth of field, then how long a subject coming straight at the camera stays inside it at three speeds.

Depth of field here uses the standard full frame circle of confusion of 0.03 mm. Three things follow from the table.

• Focus gets harder as the subject gets closer. Depth of field shrinks roughly with the square of the distance, so halving the distance from 20 m to 10 m cuts it to about a quarter. That is why the last frames of a head-on sequence are the ones most often soft.
• Every frame of a burst needs its own focus distance. At 10 frames a second there are 100 ms between frames. A cyclist at 10 m/s moves a full meter in that time, more than twice the depth of the 200mm example at 10 m.
• Slow subjects are not always easy. At 85mm, f/1.8 and 3 meters the sharp zone is 13 cm deep, about 6 cm either side of the eyes, so a person swaying slightly can drift out of it.

Put your own lens, aperture and distance into the depth of field calculator, then divide the total by your subject’s speed to get its focus budget in seconds.

AF-C compared with single-shot AF and automatic switching

Most cameras offer three autofocus modes. The labels differ between makers but the behavior is the same.

The automatic mode has to notice movement before it switches, so it can be late on a subject that starts suddenly. If you know whether the subject will move, choose the mode yourself. The single-shot mode and its recompose technique are covered in focus modes: AF-S and One Shot.

Release priority or focus priority

A second setting, normally in the autofocus or custom menu, decides what happens when you press the shutter button fully.

• Focus priority. The camera will not take the picture until it judges the subject to be in focus. Single-shot AF normally works this way. You get fewer out-of-focus frames, but the camera can refuse to fire at the moment you wanted.
• Release priority. The camera takes the picture the instant you press, focused or not. Continuous AF often defaults to this, or to a balanced option between the two. You never miss the moment, but some frames will be soft.

Some cameras let you set the first frame of a burst and the later frames separately. A sensible start is focus priority, or the balanced option, for the first frame, and release priority for the rest. With focus priority on every frame, the burst stalls whenever the camera is unsure, which is often exactly when the action peaks. A soft frame in a sequence costs nothing; a burst that stops at the peak costs the picture.

AF area modes to pair with continuous AF

The focus mode decides when the camera focuses. The AF area mode decides where it looks. Continuous AF on its own does not move the focus point to follow the subject; it refocuses on whatever is under the active area. Following the subject across the frame is the area mode’s job, described in AF tracking.

• Single point. One small point that you keep on the subject. The most precise option and the best at ignoring a busy background, but it asks for steady handling. Use it for predictable paths and for anything partly hidden.
• Zone or group. A cluster of points that act together. It forgives aiming errors, so it suits erratic subjects against a plain background. It fails when something closer than the subject enters the zone, because many cameras favor the nearest detail.
• Tracking. You start with a point on the subject and the camera moves the point to follow it, by color, pattern and distance. Works best when the subject stands out from its surroundings.
• Subject detection. The camera recognizes eyes, faces, bodies, animals, birds or vehicles, depending on the model, and focuses on them. Eye detection AF is the best-known form. It needs the subject large enough in the frame to be recognized.
• Wide or automatic area. The camera chooses from the whole frame, and without subject detection it tends to pick whatever is closest or most contrasty.

Where a zone covers subjects at different distances, some cameras let you choose which one wins: the nearest, the one nearest the center of the zone, or an automatic choice. Front priority suits a single subject against a far background. Center priority helps when foreground clutter, such as a fence or grass, keeps stealing the zone. Many cameras also have a setting for how readily focus hands over from one point to its neighbors as the subject drifts inside a zone or tracking area. A quick handover keeps up with an erratic bird; a slow one stops focus wandering onto a nearby branch.

A bird in flight is the classic case. Even when it seems to cross the frame, it climbs, drops and banks toward or away from you. Against open sky a large zone or bird detection can hold it, because there is nothing else in the zone to choose by mistake.

Which focus area suits which kind of subject? How to choose focus points covers focus area modes, the focus and recompose technique, and point selection by genre.

Tuning AF-C: blocked subjects, sudden speed changes and presets

Many cameras offer two tuning settings for continuous AF. The names differ; the ideas are the same.

Tracking sensitivity: what happens when something blocks the subject

Sooner or later a referee, a branch or another player passes in front of the subject. A setting called tracking sensitivity or blocked shot response decides what happens, on a scale from locked on to responsive. In effect it sets how long the camera keeps faith in its subject before it accepts something at a very different distance as the new target: from a fraction of a second to about a second, depending on the camera.

• Slow or locked on. A brief obstruction is ignored. The cost is a slow reaction when you swing to a new subject, or when the point slips onto the background and you bring it back.
• Fast or responsive. The camera refocuses almost at once on whatever is under the active area. Good for switching subjects and for subjects that appear suddenly; the cost is that anything crossing in front steals focus.

Leave it in the middle until you have a specific problem. Move toward locked on for team sports and wildlife in vegetation, toward responsive when nothing can get in the way. Locked on is not simply better: at its slowest it can cling to the background after you have missed the subject.

Acceleration tracking: subjects that stop and start

Prediction works best at a steady speed, because the camera can extend the line it has measured. A tennis player who sprints and stops, or a dog that bolts from a standstill, breaks that line. A setting called acceleration and deceleration tracking or speed tracking sensitivity decides how quickly the camera abandons its prediction when speed changes. Set high, it reacts to sudden starts and stops. Set low, it ignores jolts and gives steadier focus on an even pace, such as a car on a straight.

Presets: cases and sets

Many cameras bundle these settings into presets, called cases or sets, each described by a kind of subject: steady movement, often blocked, appears suddenly, erratic speed. Pick the one that matches your subject, and change one value at a time only when you can name the problem you are fixing.

Back-button focus and burst shooting with AF-C

One mode for moving and still subjects

Back-button focus moves autofocus from the half-press of the shutter button to a button under your thumb, usually marked AF-ON. With continuous AF it gives you both behaviors without a menu:

• Hold the back button and the camera tracks continuously, exactly as AF-C should.
• Press and release it and focus stays at the last distance, so you can recompose freely.
• Lift your thumb as an obstruction passes and press again once it has gone. Focus stays where the subject was, whatever the tracking sensitivity is set to.

How do you move focus off the shutter button? Back button focus covers what it is, how to set it up, and the mistakes people make while getting used to it.

How focus behaves during a burst

In burst mode with continuous AF, the camera measures and adjusts focus between frames, and each frame is a separate prediction.

• The fastest frame rate does not always give the most sharp frames. On some cameras the top speed fixes focus at the first frame or takes fewer readings. The manual says which drive speeds keep full autofocus.
• Expect some misses. A head-on sequence will rarely be sharp in every frame, and the focus budget table shows why. That is the reason to shoot the sequence.
• Short bursts beat long ones. Three to six frames around the peak of the action keep the buffer from filling and leave you ready for the next moment.

Continuous AF in video

In video, continuous AF is the normal way to autofocus. There is no shutter release to prioritize, so the settings are how fast focus moves and how readily it switches subjects. A slow, smooth transition looks deliberate; a fast one looks like hunting. Settings for both are covered in camera settings for video.

Suggested setups by situation

These are starting points. Names and options differ between cameras, so match the idea, not the exact label.

The portrait row can look out of place, but a person standing for a portrait is not really still, and neither are you. With 13 cm of depth of field at 85mm, f/1.8 and 3 meters, continuous AF with eye detection corrects for small sways right up to the exposure.

For panning, distance changes less than head-on but still changes as the subject passes, so hold a small area on one spot, such as a helmet, while the background streaks. Shutter speeds for field and court sports are listed in best camera settings for sports photography, and for animals and birds in best camera settings for wildlife photography.

When continuous AF is the wrong choice

• Static subjects on a tripod. With nothing to follow, continuous AF can make small corrections that leave one frame slightly different from the next. Single-shot AF focuses once and holds. Manual focus with a magnified view is more exact still.
• Very low light. In dim light each reading is less certain, so continuous AF can hunt, and many cameras keep the AF-assist lamp off in this mode. Single-shot AF on a contrasty edge has a better chance of locking.
• A subject on a fixed path. If you know where the action will happen, such as a jump or a finish line, focus manually on that spot beforehand and fire as the subject arrives. Setting focus to a distance in advance, and the depth of field it relies on, is covered in zone focusing.

If the lens racks through its whole range when it loses the subject, a focus limiter set to the far range shortens the search.

Is the camera hunting, or locking onto the wrong thing? How to fix common autofocus problems covers a camera that hunts and will not lock, and a camera that locks on the wrong thing.

Common mistakes with continuous autofocus

• Focusing on a face, then recomposing with the shutter half-pressed in AF-C. The camera refocuses on whatever the point lands on, usually the background. Fix: move the AF point onto the subject, switch to single-shot AF, or use back-button focus and release the button before you recompose.
• Starting autofocus at the moment of the shot. The camera has no motion history, so the first frames are guesses. Fix: pick the subject up early and hold focus on it for a second or two before the action peaks.
• Expecting AF-C to move the point for you. In single point or zone mode, the subject slides out from under the active area and the camera focuses on the background. Fix: keep the point on the subject with the camera, or choose a tracking or subject detection area mode.
• Letting the camera choose from the whole frame against a busy background. It selects the nearest or most contrasty object. Fix: use a single point or a small zone, or turn on a subject detection mode that matches the subject.
• Blaming autofocus for motion blur. A subject smeared in one direction means the shutter speed was too slow. Fix: raise it first, then judge the focus. Where that blur comes from is explained in motion blur.
• Leaving the blocked-subject response at its fastest setting in a crowd. Every passing player takes the focus. Fix: move the setting toward locked on, or, with back-button focus, lift your thumb as the obstruction crosses.

Try this: single-shot against continuous on an approaching subject

You need a camera or phone that lets you choose continuous AF, and something coming toward you at a steady pace: cyclists or joggers on a path, cars seen from a safe spot on the sidewalk, or a friend walking briskly. It takes ten minutes. Use your longest focal length, the widest aperture, and 1/1000 second or faster with auto ISO, so any softness is focus, not motion blur.

1. Set single-shot AF and a single point. As a subject approaches, focus once, keep the half-press, and take five frames as it comes closer.
2. Switch to continuous AF with the same single point. Pick the next subject up early, hold focus on it for two seconds, then shoot a five-frame burst while keeping the point on it.
3. If your camera has a tracking sensitivity setting, stand so that a post or tree briefly hides each subject. Shoot one pass at the fastest setting and one at the slowest.

How to judge it: view every frame at 100 percent and look at the eyes, a helmet or a number plate. In the single-shot set, the first frame should be sharp and each later one softer. In the continuous set most should be sharp, with any misses near the end, where the subject was closest and the depth of field thinnest. In the third pass, the frames just after the obstruction show which setting found the subject again.

Want to check you can tell the focus modes apart? Try the camera focus and autofocus quiz.

Frequently asked questions about AF-C

Is AF-C the same as AI Servo?

Yes. They are different makers’ names for continuous autofocus. Both keep refocusing while the focus button is held and both predict where a moving subject will be at the moment of exposure.

Should I leave my camera in AF-C all the time?

You can if you use back-button focus, because releasing the button holds focus just as single-shot AF would. If the shutter button controls focus, switch to single-shot AF for still subjects and whenever you focus and recompose.

Why are my AF-C photos blurry?

There are three usual causes. The shutter speed was too slow and the blur is motion, not focus. The active area was on the background or on something in front of the subject. Or the camera is set to release priority and fired before focus had caught up, which is normal for a few frames in any burst.

Why does my camera not beep in AF-C?

The beep signals that focus has been achieved and locked. Continuous AF never locks, so most cameras stay silent in that mode. It does not mean the camera has failed to focus.

What is the difference between AF-C and AF-A?

AF-C is always continuous. AF-A, or AI Focus, begins as single-shot AF and switches to continuous only if the camera decides the subject is moving. That decision can be late, so choosing AF-C directly is more dependable for action.

Is AF-C less accurate than AF-S on a still subject?

On older cameras with a separate focus sensor it could be slightly less consistent, which is where that advice comes from. With on-sensor focus and eye detection, continuous AF is often more accurate for a handheld portrait, because it corrects for small movements up to the exposure. On a tripod with a truly still subject, single-shot or manual focus is simpler.