Continuous autofocus, abbreviated AF-C and called AI Servo on Canon cameras, is an autofocus mode in which the camera keeps refocusing for as long as focus is activated, usually while the shutter button is half-pressed or a back button is held. In photography, it is the mode for moving subjects: instead of locking focus once, the camera continuously measures distance, predicts where the subject will be and drives the lens to follow it.
Continuous autofocus is the counterpart of single autofocus (AF-S or One Shot), which focuses once and locks. The difference is not the focusing technology itself but the behavior over time: AF-C never locks, and by default it allows the shutter to fire even if focus is not yet confirmed, because a moving subject cannot wait.
How continuous autofocus tracks a moving subject
In AF-C the camera runs a closed control loop: measure the focus error, move the focusing group, measure again. With phase detection AF, each measurement gives both direction and approximate amount of defocus, so the lens can be driven quickly without hunting. Contrast detection alone is less suited to AF-C because it must move the lens back and forth to find peak sharpness.
A key element is predictive focus. Between the moment the camera measures and the moment the shutter opens there is a delay of tens of milliseconds. For a runner approaching at speed, the subject moves noticeably during that time, so the camera estimates the subject’s speed and acceleration from successive measurements and focuses where the subject will be at the moment of exposure, not where it was.
Modern cameras add subject tracking, which follows the subject across the frame, and subject detection for eyes, faces, bodies, animals, birds or vehicles. This tells the AF system not only how far away the subject is but which part of it to keep sharp, usually the nearest eye.
Continuous autofocus compared with other focus modes
Cameras usually offer three or four focus modes, and choosing the right one depends on whether the subject distance changes during shooting. The table compares them.
| Mode | Behavior | Best suited for |
|---|---|---|
| Continuous AF (AF-C, AI Servo) | Refocuses constantly, does not lock | Sports, wildlife, children, vehicles |
| Single AF (AF-S, One Shot) | Focuses once and locks | Landscapes, architecture, posed portraits |
| Automatic AF (AF-A, AI Focus) | Switches to continuous if motion is detected | Casual shooting, beginners |
| Manual focus | Photographer turns the focus ring | Macro, astrophotography, video pulls |
Many experienced photographers now use AF-C as the default for almost everything, because modern tracking holds a static subject steadily as well. Single AF remains useful when the photographer wants guaranteed focus confirmation before the shutter fires, or when recomposing after focusing.
Priority settings and tracking sensitivity
Continuous autofocus is shaped by several custom settings that decide how the system reacts when the situation becomes ambiguous. The most important are listed below.
| Setting | Options | Effect |
|---|---|---|
| Release priority | Release, focus or balanced | Whether the shutter waits for focus |
| Tracking sensitivity | Locked on to responsive | How quickly AF switches to a new subject |
| Acceleration tracking | Steady to erratic speed | How the system handles sudden speed changes |
| AF area mode | Single point, zone, wide, tracking | Where the camera looks for the subject |
| Subject detection | Human, animal, bird, vehicle, off | Which subject type the camera prioritizes |
Tracking sensitivity is the setting most often misunderstood. A locked-on setting keeps focus on the original subject when something briefly passes in front, such as a referee crossing a player. A responsive setting jumps quickly to a new subject at a different distance, which suits scenes where the photographer intentionally switches targets.
Using continuous autofocus in practice
The most common way to use AF-C effectively is to combine it with back-button focus. Focus is moved from the shutter button to a rear button, so the photographer can hold the button to track, release it to stop, and shoot independently. This makes AF-C behave like single AF when needed without switching modes.
The table below gives typical starting configurations. Exact values depend on the camera, lens and subject.
| Subject | AF area | Typical settings |
|---|---|---|
| Field sports | Zone or tracking with body detection | Locked-on sensitivity, release priority |
| Birds in flight | Wide or tracking with bird detection | Responsive acceleration, high burst rate |
| Running children | Wide with eye detection | Balanced priority, 1/500 s or faster |
| Motorsport | Zone with vehicle detection | Steady acceleration, panning technique |
| Event portraits | Single point or eye detection | Focus priority for confirmed sharpness |
Shutter speed still matters. AF-C keeps the subject in the focal plane but does not freeze motion; a fast shutter speed, usually 1/500 s to 1/2000 s for action, prevents motion blur that can be mistaken for missed focus.
Common mistakes and limitations
- Focusing and recomposing in AF-C fails, because the camera refocuses on whatever lies under the new AF point.
- Release priority produces soft frames if the system has not caught up; photographers must shoot bursts and select the sharpest.
- Low light and low contrast slow down AF-C; performance is specified down to a certain EV, often around -3 to -7 EV.
- Slow lenses limit tracking: older focus motors and small apertures reduce AF speed and accuracy.
- Busy backgrounds can pull wide-area tracking away from the subject, especially with detection switched off.
Understanding these limits avoids blaming the camera for errors in technique. Most missed shots in AF-C come from the wrong area mode, a shutter speed that is too slow or a subject that the AF point never covered.
Put simply
Continuous autofocus is a camera mode that keeps focusing again and again while the photographer holds the button. It is made for things that move, such as a dog running toward the camera or a child on a bicycle.
Imagine trying to keep a moving car sharp in binoculars. You keep turning the focus wheel as it comes closer. Continuous autofocus does the same thing automatically, many times per second. It even predicts where the subject will be a split second later, so the photo is sharp at the moment it is taken.
In practice
A photographer covers a youth football match from the sideline with a 70-200 mm lens. In single autofocus, many frames came out soft when players ran toward the camera, because focus locked on a distance the players had already left.
Switching to continuous autofocus with body detection and zone AF, back-button focus and a shutter speed of 1/1000 s changes the result. The photographer holds the button as a striker sprints toward goal and shoots a burst of ten frames. Thanks to predictive tracking, nine of the ten frames are sharp, including the moment of the shot on goal. The locked-on tracking setting keeps focus on the striker even when a defender briefly crosses the frame.