An autofocus (or AF) is a device connected to an optical system. The device operates in two steps:

• distance measuring
• focus adjustment

There are several ways to measure distance: mechanical (enlargers), sound-waves (some Polaroid cameras), IR light (some compact cameras, early video camcorders), phase detection (SLR cameras), contrast detection (video-cameras, digital-image cameras).

Focus adjustment is mainly controlled electronically (except from the mechanical direct adjustment in some enlargers). The data collected from the distance measuring are calculated in an electronic unit which activates a mechanical or electromechanical system to adjust the focus accordingly.

Sound-waves and IR light are active systems, as they measure the delay in the reflection of the sound- or light-waves. These systems are fooled when photographing through windows. Passive systems such as phase-detection and contrast-detection may focus correctly through windows. These systems measure continuously as the focus changes until the lens has focused.

The speed and accuracy of autofocusing can be very good; normally the performance is better than could be achieved manually. Modern cameras measure several areas of the picture and decide where the subject is. Some cameras are able to decide if the subject is moving towards or away from the camera, what speed it has, at what rate it accelerates, and can follow the subject as it crosses the image.

Simple AF systems rely on a single focus sensor. Advanced systems consist of an array of focus sensors. The Canon EOS 3 has as many as 45 sensors, each accessible with their unique eye-control, to focus the camera on the photographer's subject. Most modern SLR cameras use data from the AF system to assist in the light measuring.