Single vs Continuous Autofocus: AF-S, AF-C, and AI Servo

Continuous autofocus is a focus mode in which the camera keeps adjusting focus while you hold the shutter button halfway, so it can follow a moving subject. Single autofocus locks focus once and stays there. Canon calls them One-Shot and AI Servo, Nikon and Sony call them AF-S and AF-C, and Fujifilm uses AF-S and AF-C as well.

Why It Matters

Focus is the most common reason a photo fails. A sharp frame needs the focus point to be on the subject at the moment of exposure. Single autofocus is excellent for still subjects, but a person walking toward you moves out of the focus plane before the shutter fires.

Continuous autofocus solves that by tracking. Choosing the right mode before you shoot is one of the best ways to increase the number of sharp frames.

Beginner When To Focus Once And When To Keep Tracking

Look for a focus mode switch or menu setting. It may be a switch on the camera body, a button with a dial, or a menu item named Focus Mode. The names differ, but the choices are the same.

How Single Autofocus Behaves

You half-press the shutter button, the camera focuses, a beep or a green dot confirms it, and the focus stays locked while you hold the button. Many cameras will not take the picture until focus is confirmed. This is called focus priority and it is what you want for a landscape, a still life or a portrait of a seated person.

How Continuous Autofocus Behaves

The camera keeps refocusing while you hold the half press. If the subject moves closer, the focus follows. Many cameras allow a picture to be taken at any moment, even if the subject is not sharp yet. That is release priority, and it helps you catch the moment you wanted.

Which Mode For Which Subject

Steps To Try

1. Set the camera to single autofocus and focus on a still object, then recompose and watch focus stay put.
2. Switch to continuous autofocus and follow a person walking toward you, keeping the half-pressed button down.
3. Take a burst of five or ten frames and check how many were sharp.
4. Try the same walk in single mode and compare.

Common Mistakes

• Using single autofocus on a moving subject and wondering why the face is soft.
• Releasing the half press between shots in continuous mode, which makes the camera start over each time.
• Using continuous mode with focus and recompose on a still subject, where the camera refocuses on the background.
• Forgetting to turn the mode back, then missing a quiet portrait because the camera hunts.

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Advanced Predictive Tracking, Release Priority, And Tracking Sensitivity

How Predictive Tracking Works

When the subject moves toward or away from you, the focus distance changes over time. The camera measures several focus readings in rapid succession, estimates the speed of the subject, and predicts where it will be at the moment the sensor is exposed. It then drives the lens to that predicted distance. This matters because the shutter has a short delay, and the lens needs time to move. Prediction works best when the subject changes speed smoothly. Sudden stops and starts defeat it. The sensors behind this are covered in phase detection autofocus.

Release Priority Versus Focus Priority

Most cameras let you choose what the shutter does when focus is not confirmed. Focus priority holds the shutter until focus is achieved, so you may lose a frame but every picture is sharp. Release priority fires immediately. Balanced settings mix the two. For action, many photographers use release priority in continuous mode so the burst never stalls, then accept that some frames are soft. For portraits, use focus priority in single mode so you never get a blurred keeper.

Tracking Sensitivity And Subject Switching

Many cameras have a tracking sensitivity or lock-on setting. A slow setting keeps focus on the current subject when something passes in front, which suits a game where players cross. A fast setting jumps to a new subject quickly, which suits erratic subjects such as birds and dogs. Some cameras also have settings for accelerating or decelerating subjects. See AF tracking sensitivity for ways to tune this.

Combine With The Right Area Mode

Continuous mode needs an area mode to tell the camera where to look. A single small point demands precision from you, while a zone or tracking mode with subject detection lets the camera hold on. Choose them together, see autofocus area modes and subject detection autofocus.

Drive Mode And Buffer

Continuous focus pairs with continuous shooting. At 10 or 20 frames per second, the camera focuses between frames, so the speed of the lens motor matters. Slower lenses limit how many frames in a burst are in focus. Learn more in drive modes and burst shooting.

Back-Button Focus

You can separate focus from the shutter. Assign focusing to a rear button, then hold it for continuous focus and release it to lock. This lets you switch between tracking and locking without changing modes. See back-button focus.

Try It

1. Set AF-C or AI Servo, release priority, and a wide zone area. Photograph a friend walking toward you from 10 meters away.
2. Repeat with single autofocus and count the sharp frames in each series.
3. Change tracking sensitivity to its slowest and fastest values, and photograph a person passing behind a pole.
4. Try focus priority with continuous mode and compare the number of frames recorded.

Common Questions

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

AF-S focuses once and locks while the shutter button is held half pressed. AF-C keeps refocusing while the button is held. Choose AF-S for still subjects and AF-C for moving ones.

Is AI Servo the same as AF-C?

Yes, in practice. AI Servo is Canon’s name for continuous autofocus. It does the same job as AF-C on other brands, though settings and tracking options differ between models.

Can I leave my camera on continuous autofocus all the time?

Yes, and many photographers do. It works for still subjects as well, but the camera can refocus after you recompose. For precise focus-and-recompose work, single mode or back-button focus is safer.

Why does continuous autofocus miss in low light?

Autofocus needs contrast and light to measure distance. In dim scenes the camera hunts or lags behind. Use a wider aperture lens, a brighter area, an AF assist light, or switch to a larger focus area.

Related Lessons

• Autofocus Area Modes to pick where the camera looks.
• Back-Button Focus to switch between tracking and locking.
• AF Tracking Sensitivity to tune how tightly the camera holds on.
• Phase Detection Autofocus to see how the camera predicts movement.
• Drive Modes And Burst to pair tracking with continuous shooting.