LESSON 52
Focus modes and back-button focus
Single, continuous, and the thumb trick.
IN THIS LESSON
- How autofocus finds focus
- Single and continuous
- Choosing the area
- In practice: back-button focus
- In practice: when to focus by hand
How autofocus finds focus
An autofocus system uses a sensor, a controller and a motor to focus on a point or area that you or the camera choose. Most systems are passive and work in one of two ways. Phase detection splits incoming light into pairs of images and compares them, and lenses built for it move quickly and directly to the new focus position. Contrast detection sweeps the focus back and forth and stops where the image shows the most contrast. Hybrid systems combine methods to cover each one's weak spots. Mirrorless cameras once relied on contrast detection, but phase detection is now the norm on most of them, which gives much better tracking.
Single and continuous
Cameras offer at least two modes. In single autofocus, the camera focuses once when you half-press and then locks, which suits still subjects and lets you focus and recompose. In continuous autofocus, the camera keeps adjusting while you hold the button, and some systems can predict a subject's speed and direction toward or away from the camera. Canon calls this AI Servo; Nikon calls it continuous focus. Speed also depends on the lens. The source notes that f/2 to f/2.8 is generally considered best for focusing speed and accuracy, and that most consumer cameras focus reliably only with lenses of f/5.6 or wider.
Choosing the area
Modern cameras let you pick a single point, a zone of points, or let the camera choose across the frame, often with face, eye or animal detection. A single point gives you control when the subject is small or behind clutter. A zone suits a subject moving unpredictably inside one part of the frame. Full automatic with eye detection works well for portraits and many action scenes, but it may pick the wrong person or the nearest object. The source also describes trap focus: the camera is set to fire only when something moves into the plane you focused on, which is useful for wildlife and sports.
In practice: back-button focus
This section is working practice and the name is the common one, not a formal standard. Many photographers move focusing off the shutter button and onto a button under the thumb, often labelled AF-ON. The shutter then only takes the picture. With continuous mode, holding the thumb button tracks a moving subject; letting go holds focus where it is, so you can recompose a still subject without the camera refocusing. One mode now does the work of two. Look for it in your camera's menu under custom controls. Give it a week before you judge it; the first days feel clumsy.
In practice: when to focus by hand
Also practice. Autofocus struggles in very low light, with low-contrast surfaces such as fog or a blank wall, through glass, wire or branches, and with tiny subjects at macro distances. Switch to manual focus, magnify the view on the screen, and turn the ring until fine detail snaps into place. Many cameras can also show focus peaking, which outlines sharp edges in colour. For stars, landscapes and repeated shots from a tripod, manual focus is often faster than fighting the system.
Try it today
- 01Set single autofocus and one focus point, then focus on a still object and recompose.
- 02Switch to continuous autofocus and follow a person walking toward you.
- 03Assign focus to the thumb button and turn it off on the shutter button.
- 04Shoot a still subject and a moving one using only the thumb button for focus.
- 05Try manual focus with magnification on a low-contrast surface.