Just about all digital cameras have autofocus systems, and while there’s a wide range of different autofocus (AF) technologies, the basic principles of how autofocus works is the same across all of them and boils down to two things: AF modes and AF points.
AF mode: WHEN the camera focuses
If you’re taking one picture at a time, you’ll want the camera to focus immediately before you take the picture. This uses the camera’s S-AF (single-shot AF) mode.
In this mode the camera will typically focus the shot the moment you half press the shutter release. It will confirm focus with an illuminated symbol in the viewfinder or on the LCD screen and with an audible ‘beep’. You then press the shutter button the rest of the way to take the picture.
You can also ‘lock’ the focus if you need to reframe the shot before taking the picture, either by keeping the shutter button half-pressed while you reframe (this works on most but not all cameras), or by using the camera’s AE-L/AF-L button (which can also be used to lock the exposure).
If you’re shooting with the camera’s continuous shooting or ‘burst’ mode, you’ll want it to focus differently. Here, you want the camera to keep focusing all the time you’re holding down the shutter button. This is C-AF (continuous AF) mode.Camera makers sometimes call this something different – Canon calls it ‘Servo AF’.
Some cameras can automatically distinguish between static and moving subjects and switch AF modes automatically. This automatic mode is a third option in the list.
AF points: What the camera focuses ON
In the early days of autofocus, cameras could only focus on a single point in the centre of the frame. Nowadays, they have AF points (focus points) all over the frame.
Different cameras offer different AF point selection modes, but there are some options that are common across all cameras.
In single point AF mode, you choose the focus point yourself. You can just the center AF point (this may be a separate option of its own), or you can move the focus point around the frame if your main subject is off-center.
Area AF mode is fully automatic. Here, the camera checks all its focus points and selects one automatically. It will usually pick the object closest to the camera to focus on, which is typically what you’d want to focus on anyway.
Some cameras offer automatic AF point selection over a smaller area so that it’’s a little more selective, typically called Zone AF.
Eye AF is an emerging technology that can automatically identify human eyes in a scene and focus on them – it’s ideal for portrait and people photography.
Many cameras also offer subject tracking AF modes for following moving subjects – this is getting into more specialized continuous shooting applications.
Other autofocus options
Camera makers will quote all sorts of specifications and jargon around autofocus systems.
Autofocus speed is one of the popular ones. AF time is quoted in fractions of a second and makers are in a constant race to be the fastest. In reality, the AF speed will depend on many factors, including the subject distance and the lens you’re using, so these figures are always ‘best-case’ figures.
The number of AF points is another obvious selling point, but the AF coverage is equally important. This is how much of the image area is covered by the AF system. The best systems can cover practically the whole frame, but some only cover the central region.
Autofocus systems currently use two different technologies, which are also often highlighted in camera specifications. Contrast AF is an older system that’s precise and effective on cameras with smaller sensors, but can prove slow on DSLRs and mirrorless cameras. Phase detection AF is a faster but more complex system used for DSLR autofocus systems but now adapted for use directly on image sensors. Most mirrorless camera makers now offer phase-detection AF.
Some cameras have an AF Assist lamp on the front to help them focus in low light on objects close to the camera. More advanced cameras may have an AF Fine Tune feature to improve the focus accuracy with specific lenses, though this is an issue more likely to affect DSLRs than mirrorless cameras because the AF sensor in a DSLR is separate to the imaging sensor.