Autofocus (AF) is a focusing system found in modern cameras which can automatically control the optics in a lens to bring a subject into focus. Manual focus, by comparison, is when the photographer controls their lens' focus by hand, usually turning a focus ring on the lens. Modern autofocus can quickly focus on, and track, the desired subject with the press of a dedicated AF button, or with a "half-press" of the shutter release button.
Technical explanation of Autofocus
Autofocus (AF) technology first came into being in very crude forms rather slowly during the 1960's and 1970's, and was only widely adopted by major camera companies in the 1980's. Today, most consumer and professionally oriented lenses and cameras offer autofocus, with only a few speciality lens companies offering lenses that must always be focused manually.
Different types of autofocus technologies are used today, and the AF system included in your camera will depend on what type of camera system it is. Generally speaking, most autofocus systems fall into two different categories: autofocus that is performed by the image sensor itself, and focusing that is done by a separate AF module.
Optical Phase-Detect Autofocus in SLR Cameras
If a camera has a mirror and a shutter, it will use an autofocus technology known as phase-detect autofocus. This type of AF system usually relies on an AF module which is separate from the image sensor itself. This AF module focuses the lens by using light that is bounced onto it from another mirror hidden behind the main one.
This AF technology has become highly accurate, and high-end professional cameras have impressive, intelligent phase-detect AF systems. However using a mirror and a separate autofocus module has one inherent flaw: it isn't actually using the sensor itself to verify focus.
To compensate for minuscule (but consistent) discrepancies in a camera or lens' autofocus precision, many modern cameras offer an autofocus fine-tuning system. (See AF Microadjustment)
Sensor-Based Autofocus in Mirrorless Cameras
Cameras that don't have a mirror (also, SLR cameras that have a "live view" mode) use the image sensor itself to perform autofocus. This is usually performed by a method known as contrast-detect AF, which was previously inferior at both low-light autofocus performance, and continuous subject tracking.
However, some recent mirrorless cameras (and live view DSLRs) have developed a method of using the image sensor itself to perform phase-detect autofocus. Other cameras have gained a hybrid autofocus system, that performs both phase-detect and contrast-detect autofocus on the image sensor.
Both of these newest autofocus systems can be extremely accurate and less prone to errors, especially in optimal (bright) lighting conditions. The newest iterations of these technologies are finally rivaling the autofocus performance of even the best traditional optical phase-detect AF systems.
Other Autofocus Related Terms And Definitions
Autofocus systems have numerous features and options, some of which are sometimes confusingly named. Here is a brief list of some of the common terms used in Autofocus systems:
- Focus Points: The individual points around the viewfinder where autofocus is actually performed. Most cameras allow you to either select an individual focus point and move it around the viewfinder, or let the camera automatically pick whichever focus point it thinks is best.
- Cross-Type Focus Points: A type of focus point that actually consists of two phase-detect focus points in one, which allows the focus point to detect image detail of any shape. By comparison, a non-cross-type AF point is only capable of detecting image detail that runs in a certain direction.
- Single Focus: the AF mode that locks focus on your subject once and then stops, allowing you to wait for a candid moment or re-compose your image before clicking the shutter. Also known as One Shot Focus.
- Continuous Focus: the AF mode that continuously tracks a subject, allowing you to maintain focus on a moving subject for as long as you hold down the AF button. Also known as Servo Focus, or AI-Servo
- Dynamic Focus: an autofocus mode that allows the camera to move / change the active focus point, in order to track a subject if it moves around the viewfinder.
- AF-ON: a customization that allows the photographer to activate autofocus using a different button other than the shutter release, and if desired, dis-allow the shutter release button from performing autofocus at all. Some cameras have a dedicated AF-ON button, on other cameras the function must be custom-assigned to another button. Also called Back-Button Focus.