2.4. Autofocus

How Autofocus Works

Modern autofocus, often called AF, measures distance to your subject and moves the lens so that light rays converge sharply on the sensor. The basic idea is simple, but the implementation differs between camera types.

In DSLR cameras, autofocus usually relies on a dedicated phase-detection module. Light coming through the lens is split into two beams before it reaches the viewfinder. The camera compares these beams. If they do not align, the subject is out of focus, and the camera knows both which direction and how far to move the focusing elements. This makes phase detection fast and well suited to tracking moving subjects.

Mirrorless cameras and many newer DSLRs use on-sensor autofocus. Here, some or all pixels on the imaging sensor are used for focusing. Two main methods are common. Contrast-detection AF measures image contrast directly on the sensor. The camera moves focus until the image contrast is highest. This is accurate for stationary subjects but can be slower, because the lens has to hunt back and forth to find the contrast peak. On-sensor phase detection uses special pixels that receive light from slightly different parts of the lens. Just like the DSLR AF module, the camera can calculate which direction and how far to move the lens, which allows fast, decisive focusing.

Most modern mirrorless cameras use a hybrid system that combines both. Phase detection gives a quick, approximate focus position, and contrast detection fine tunes it. This combination is the reason many current cameras focus quickly and with high accuracy, even in low light.

Autofocus works by driving a motor in the lens. When you half-press the shutter or press a dedicated AF button, the camera measures focus, commands the lens to move, and confirms focus when it reaches the target distance. If you understand that the camera is always trying to solve the problem of distance to subject, it becomes easier to choose the right AF mode for different situations.

Single Autofocus

Single autofocus is designed for subjects that are not moving significantly. Different brands use different names for it, such as AF-S, One-Shot AF, or Single AF. In this mode, the camera focuses once when you start AF, then locks that focus distance until you release the button or take the shot.

The basic workflow is to choose an autofocus point, place it over the part of the subject you want sharp, then half-press the shutter. The camera drives the lens until that point is in focus, then usually confirms with a beep or a visual indicator. After that, as long as you keep the button half-pressed, the focus distance does not change, even if the subject or the camera moves.

Single AF works well for portraits of people who are relatively still, landscapes, architecture, product shots, and any situation where distance to subject is stable. It is also helpful in low light, because the camera can take its time to find focus without constantly adjusting.

One common technique with Single AF is focus and recompose. You place the AF point over the subject, focus, then, while holding the focus, reframe the image so the subject sits where you want in the composition. This is convenient, but it can introduce slight focus errors at very wide apertures, especially if you are very close to the subject. When you care about extremely precise focus, it is safer to move the focus point itself over the final subject position instead of recomposing too much after focusing.

Single AF is predictable and stable. Use it whenever your subject will not move toward or away from the camera while you are shooting.

Continuous Autofocus

Continuous autofocus, often called AF-C, AI Servo, or Continuous AF, is built for moving subjects. In this mode, the camera keeps adjusting focus as long as you hold the AF button. It constantly reevaluates distance and updates the lens position to follow the subject.

With continuous AF, you usually keep an AF point or AF area over your subject while it moves. The camera samples focus many times per second, predicting where the subject will be at the moment of exposure. This predictive behavior is crucial when subjects are moving quickly, such as in sports, wildlife, or children running.

Continuous AF is less about a single focus confirmation and more about ongoing tracking. There is often no confirmation beep, because focus is always changing. You typically combine Continuous AF with burst shooting so you can capture multiple frames as the subject moves, increasing your chances of getting a perfectly sharp moment.

Because the camera is always updating focus, small movements of the camera or subject are less likely to ruin sharpness. However, this also means the AF system is constantly working. If you unintentionally let the AF area drift away from your subject, the camera can snap focus to the background. Good technique is vital. You must practice keeping the AF area on your subject and anticipate their movement.

Use Continuous AF whenever your subject is moving toward or away from you, or when you expect unpredictable motion. This includes sports, wildlife, street scenes with walking people, or action portraits where your subject is moving naturally.

Autofocus Points and Areas

Autofocus points are the individual focus sensors that the camera can use to measure distance. Older or simpler cameras may have only a few points, while modern cameras often have hundreds or even thousands spread across much of the frame. In practice, you do not select each point individually on cameras with very dense arrays. Instead, you choose an AF area mode, which is a grouping or behavior pattern for those points.

Common AF area modes include single-point, expanded point, zone, and automatic area. Their names vary slightly between brands, but the ideas are similar.

In single-point AF you choose one specific AF point. The camera uses that point alone to acquire focus. This gives you very precise control and is ideal for subjects that are easy to follow, such as static portraits, product shots, or detailed compositions where you must place focus exactly.

Expanded or dynamic-area AF starts with a selected point, but allows nearby points to help if the subject briefly moves away from the primary point. This is helpful for tracking subjects that move unpredictably but remain in roughly the same area of the frame, such as a bird in flight or a running athlete.

Zone AF groups a cluster of points together as a block. You move the block inside the frame, and the camera chooses which point inside that zone to focus on, usually preferring the closest or most contrasty subject. This is useful when you cannot precisely keep a single point on the subject, such as when photographing people in a crowd or fast street scenes.

Automatic or wide-area AF uses a large portion of the frame, sometimes the entire area, and lets the camera decide what to focus on. The camera often looks for faces, eyes, or the nearest object with strong contrast. This mode is convenient when you trust the camera to pick the main subject or when you are shooting casually and speed matters more than precision.

Choosing the right AF area is about control versus convenience. Smaller areas give you more control but demand better technique. Larger areas make life easier but give more decision power to the camera. Over time, you will learn which AF area pairs best with Single AF or Continuous AF for different subjects.

Subject Detection

Subject detection is an advanced autofocus feature where the camera tries to understand what the subject is, not just where there is contrast. Instead of treating every part of the scene equally, it uses machine learning and pattern recognition to identify people, animals, vehicles, or other specific subject types.

When subject detection is active, the camera analyzes the entire frame and searches for the subjects it knows how to recognize. Once it finds one, it can assign autofocus points to follow that subject automatically. For example, with human detection enabled, the camera will often ignore busy backgrounds and lock on to faces or bodies, even if they move within a wide AF area.

Different cameras may offer separate options for detecting people, animals, birds, or vehicles. When possible, it is helpful to choose the category that matches what you are shooting. This reduces confusion and helps the AF system prioritize the right shapes and movements.

Subject detection is especially powerful when combined with Continuous AF and larger AF areas. Instead of carefully keeping a single point on a moving subject, you allow the camera to choose and follow the detected subject inside the frame. This can dramatically improve your keeper rate in action, wildlife, or events, and it can also simplify portrait work, especially in dynamic situations.

Subject detection is not infallible. It can struggle in very low light, with very small subjects, or when the subject is partially hidden. It is still important to watch what the camera is doing and be ready to override it with a different AF area or manual subject choice when necessary.

Eye Detection

Eye detection is a specialized part of subject detection focused on finding and tracking eyes, usually human eyes but often also animal or bird eyes on newer cameras. Because in most portraits the eye closest to the camera should be the sharpest point, eye detection solves a common focusing challenge.

When eye detection is enabled, the camera first looks for a face, then tries to locate the eyes inside that face. It places a small focus box over one eye and continuously tracks it as the subject or camera moves. In many systems you can choose whether to prioritize the left or right eye, or let the camera decide.

This feature is extremely helpful when shooting with a shallow depth of field, such as at wide apertures like f/1.4 or f/2.0, where slight focus errors are obvious. It also helps when your subject moves slightly, laughs, or turns their head during a portrait session. Instead of you constantly moving an AF point around, the camera does the fine tracking.

Eye detection is most reliable when the eyes are reasonably large in the frame and well lit. If the subject is very small, turned far away, wearing sunglasses, or heavily obscured, the camera may fall back to face detection or even to body detection. In those cases, you might need to move closer, adjust your composition, add light, or temporarily switch to another AF method.

For static portraits, combining Single AF with eye detection can give quick and accurate focus. For moving subjects like walking people, children, or animals, Continuous AF with eye detection is often the most effective combination. It allows you to concentrate on timing and composition while the camera handles precise focus on the eyes.

Focus Lock

Focus lock is a way to hold a chosen focus distance even if you or the subject moves the AF area away from the original point. It is crucial when you want to recompose the frame after focusing, or when something temporarily passes between you and the subject.

In Single AF, focus lock happens naturally as long as you keep the shutter button half-pressed after the camera confirms focus. You can point the selected AF point at your subject, half-press to focus, then keep holding the half-press while you reframe the composition. The focus distance remains fixed until you release the button or take the photo.

Many cameras also have a dedicated AF-L button, which stands for autofocus lock. When you press AF-L, the current focus distance is held, regardless of what you do with the shutter button. This can be convenient if you want to separate focusing from exposure or if you find half-pressing the shutter uncomfortable.

A related technique is back-button focus. In this setup, autofocus is removed from the shutter button and assigned to a rear button, usually labeled AF-ON. You press AF-ON to focus, then release it and recompose freely while the focus distance remains unchanged. This effectively gives you manual control over when the camera is allowed to refocus and when it is locked.

In Continuous AF, focus lock behaves a bit differently. Because the camera is constantly updating focus, you usually lock focus by releasing the AF button or by using an AF stop control on some lenses. Once you stop AF, the lens stays at the last focused distance, allowing you to hold that focus intentionally even as the subject continues to move.

Focus lock is useful whenever you want to control focus independently of composition or timing. For example, you might lock focus on a doorway, then wait for someone to walk through it and press the shutter at exactly the right moment, knowing the focus distance is already set.

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