AF-S and AF-C

Learning goal

You will be able to choose consciously what autofocus does with the sharpness it finds: lock it once, or keep searching for it again. You will explain that single-shot focus finds sharpness on still subjects and fixes it, while continuous focus follows a moving subject and renews sharpness every moment; you will know the difference between focus priority and release priority; you will see how the two modes react differently to recomposing; you will apply the tricks of following moving subjects with continuous focus; and you will keep moving people sharp in dim light or bright sun by finding an edge to track, according to the skin's tone.

The question

In the park, you want to photograph a dog running toward you. You half-press the shutter button, see the camera focus on the dog, and press. In the first frame the dog is sharp. But as the dog comes closer, the frames you take grow softer; in the last frame the dog's nose fills the frame but is blurry, while the grass behind it is sharp.

How does a moving subject stay sharp? The camera found sharpness, but then let go of it; while the dog moved, sharpness stayed at the distance where it was first found. Yet the camera also has a mode that keeps searching for sharpness again and again. In this lesson we will talk about these two focus modes, which one is useful when, and how to keep a moving subject sharp across frames.

Context

In the previous lesson you saw that the camera finds sharpness with two different methods: contrast detection and phase detection. Finding sharpness is the first half of the job. The second half is what to do with the sharpness found: find it once and leave it there, or search again as the subject moves? This decision is chosen in the shooting settings as the focus mode.

On many cameras these two modes are shown with the abbreviations AF-S and AF-C; some manufacturers use different names, but their jobs are the same. Some cameras also have an automatic mode that leaves the choice to the camera. Phones usually adjust focus continuously on their own and lock it on a point when you tap the screen. In the exposure module, movement was tied to the shutter speed: a short time freezes movement. But freezing movement isn't enough; the frozen subject also has to be in the plane of focus. The focus mode provides this second condition.

Simple explanation

How does single-shot focus work?

With single-shot focus, when you half-press the shutter button, the camera searches for sharpness once, finds it, and locks it there. This is single autofocus (AF-S). A short beep or a frame that changes color usually announces the lock. As long as you keep it half-pressed, sharpness stays at that distance; even if the subject moves, the camera doesn't search again. On many cameras this mode has focus priority: if the camera can't find sharpness, it doesn't let the shutter fire; instead of taking a blurry frame, it makes you wait.

This mode is ideal for still subjects: a portrait, a landscape, a meal on the table, a person sitting down. The locked sharpness also makes recomposing possible. You focus on the subject's eye, keep it half-pressed, then turn the camera and place the subject at the edge of the frame; sharpness stays on the eye. The trap of single-shot focus is moving subjects: once sharpness is locked, if the subject comes closer to you or moves away, the plane of focus stays in place and the subject leaves it. This is exactly the problem in the dog example.

How does continuous focus work?

With continuous focus, as long as you keep the shutter button half-pressed, the camera never locks sharpness; it follows the subject and readjusts sharpness every moment. This is continuous autofocus (AF-C). If the subject comes closer, the lens moves accordingly; if it moves away, the lens goes back. In this mode many cameras predict the subject's speed and direction, foreseeing where the subject will be at the moment you press the shutter button; this is called predictive tracking. The speed of phase detection is the basis of this tracking.

With continuous focus, most cameras use release priority by default: even if sharpness isn't perfect, they shoot when you press the shutter button, because waiting in a moving moment means missing the moment. On many cameras this behavior can be changed in the menu. The trap of continuous focus, on the other hand, is recomposing: if you turn the camera while keeping it half-pressed, it focuses on whatever newly comes under the focus point, for example the background. That is why with continuous focus, instead of recomposing, you need to move the focus point onto the subject; we will see this in the next lesson.

Which mode should you choose when?

The core of the decision is this: if the subject is still, single-shot; if it is moving, continuous focus. But the line isn't always clear. A person swaying while talking, a baby playing, a flower swinging in the wind make small movements. In situations like these, especially when working in a shallow slice of sharpness with a wide aperture, even a small back-and-forth movement can shift sharpness from the eye to the ear. Continuous focus follows these small movements too.

In the automatic selection mode, the camera looks at whether the subject is moving and chooses the mode itself. This mode offers convenience but sometimes gets it wrong; it can take a still subject for a moving one and slide to the background, or take a moving subject for a still one and lock it. That is why, if you know what you want in which situation, choosing the mode yourself is more reliable. Many photographers make continuous focus the default in every scene where movement is possible, and switch to single-shot for still subjects.

How do you keep sharpness on a moving subject?

When following a moving subject with continuous focus, a few habits make the job easier. Keep the subject under the focus point and start tracking for a while before pressing the shutter button; the camera needs a few moments to learn the subject's speed and direction. Take several frames in a row with continuous shooting; in a series, the chance of at least one frame being perfectly sharp goes up. Provide enough light; phase detection slows down as the light drops. Think about sharpness together with the shutter speed: in a blurry frame, you need to tell apart whether the problem is focus or the duration of the movement.

Tracking relies on edges, and a moving face sometimes offers few edges. A dark-skinned child running in a dim sports hall, or a light-skinned face shining in the sun, can give the camera little contrast, and tracking can slide off the face. In this case, placing the focus point on a contrasty area, for example the lettering on a jersey, the edge of the hair or around the eyes, strengthens tracking. Increasing the light, that is moving to a brighter area or raising ISO, also increases the speed of tracking. The face and eye tracking of modern cameras takes over much of this job; we will look at that separately in this module too.

Visual evidence

In the park, you are photographing a dog running toward you to catch a ball a child has thrown. First you try with single-shot focus. While the dog is twenty meters away you half-press, focus, and shoot in a burst. The first frame is sharp; but as the dog comes closer the frames grow softer, and in the last frame the dog's nose is blurry while the grass behind it is sharp. Sharpness has stayed at twenty meters.

You switch the focus mode to continuous focus and shoot the same run again. You keep the dog under the focus point, track it for a few seconds before pressing the shutter button, and shoot in a burst. As the dog comes closer, sharpness comes with it; in the last frame the water droplets on its nose and its ears flying in the air are sharp. Then, in the same park, you photograph your grandmother sitting on a bench in continuous focus and try to recompose; sharpness slides to the tree behind. You switch to single-shot, focus on her eye, lock it and set up the frame; your grandmother's eyes are sharp. Two subjects, two modes; each does its own job well.

Common mistakes

Summary

Single-shot focus finds sharpness once and locks it, usually doesn't allow a shot before sharpness is found, and is ideal for still subjects and for recomposing with locked sharpness; continuous focus follows the subject and renews sharpness every moment, can predict its speed and direction, and usually shoots even if sharpness isn't perfect, so as not to miss the moment. Single-shot for a still subject, continuous focus for a moving subject, is the basic decision; with continuous focus, the focus point is moved instead of recomposing. Because tracking a moving face relies on edges, on low-contrast faces in dim light or bright sun, putting the focus on a contrasty area such as jersey lettering, the edge of the hair or the eyes, and increasing the light, strengthens tracking.

Mini assignment

In this assignment you will compare the two focus modes on the same movement.

1. Photograph someone running or riding a bike toward you with single-shot focus, taking at least five frames in a row.
2. Shoot the same movement again with continuous focus, keeping the subject under the focus point and tracking it for a few seconds before pressing.
3. Photograph a still person by recomposing in continuous focus, then by recomposing with focus lock in single-shot, and look at where sharpness falls.
4. Photograph someone moving in a dim place; if tracking slides off the face, move the focus point to a contrasty area.

Check yourself

• Did you compare how many frames were sharp in the single-shot and continuous focus series?
• Did you see sharpness slide when you recomposed with continuous focus?
• In the dim light, was the running person's face tracked more steadily once the focus was moved to a contrasty area?

Glossary

Mini quiz

1. With single-shot focus, when is sharpness searched for?

• A) Once when the shutter button is half-pressed; then it is locked
• B) Continuously, every moment
• C) Only after the photo is taken
• D) It is never searched for

2. When a dog running toward you is photographed with single-shot focus, why do the last frames get blurry?

• A) Because the shutter speed is too long
• B) Sharpness stays at the distance where it was first found, and the dog leaves that plane
• C) Because the ISO is too high
• D) Because the dog's color looks like the background

3. What is the problem with recomposing with continuous focus?

• A) The camera switching off
• B) The exposure going wrong
• C) The colors changing
• D) The camera focusing on the background that newly comes under the focus point

4. If tracking slides off the face of a dark-skinned child running in a dim hall, what can you do?

• A) Switch to single-shot focus and lock the first frame
• B) Stop the aperture down to its narrowest
• C) Put the focus point on a contrasty area such as the jersey lettering or the edge of the hair, and increase the light
• D) Turn off the assist light

Answers

1. A — Single-shot focus finds sharpness once on the half-press and keeps it locked there while the button is held.
2. B — The locked plane of focus stays fixed; the approaching dog grows blurry once it leaves this plane, while the background can stay in focus.
3. D — Continuous focus follows whatever is under the focus point; when the frame is turned, the background that comes there is focused.
4. C — Tracking relies on edges; a contrasty area and more light give the camera a target it can follow.

Summary card

• Single-shot (AF-S): find once, lock; still subject and recomposing.
• Continuous (AF-C): follow the subject, renew sharpness every moment; moving subject.
• With continuous focus, don't turn the frame, move the focus point.
• Start tracking before you press, shoot in a burst, increase the light.
• On a low-contrast moving face, put the focus on a contrasty area.

Bridge to the next lesson

We said that with continuous focus, instead of turning the frame, you need to move the focus point onto the subject. But where does the focus point sit, and how do you steer it? When the camera chooses a point on its own, it usually focuses on the nearest or brightest thing. The next lesson describes focus areas, and why you should choose the focus point yourself instead of leaving it to the camera.