Autofocus (or AF) is a feature of some optical systems that allow them to maintain correct focus on a target. See also list of optical topics. ...

General

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. The Canon EOS (Electro-Optical System) film and digital SLR camera system was introduced in 1987 with the Canon EOS 650. ...

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.

Different AF systems

There are two different approaches to systems for automatic focusing:

• active systems
• passive systems

Active systems

These systems usually work in two steps:

• distance measuring (separated from the optical system)
• focus adjustment to the optical system

There are several ways to measure distance: sound-waves (some Polaroid cameras), IR light (some compact cameras, early video camcorders). Sound-waves and IR light are emitted from the camera, and by measuring the delay in the reflection of the sound- or light-waves, distance can be calculated. Polaroid® (a trademark of the Polaroid Corporation) is the name of a type of synthetic plastic sheet which is used to polarise light. ... A camera is a device used to take pictures (usually photographs), either singly or in sequence, with or without sound, such as with video cameras. ... IR may mean: Independent-Republican Party of Minnesota the former name of the Republican Party of Minnesota. ... Video (from Latin, I see) is the technology of processing electronic signals for representing moving pictures. ... 8mm Camcorder mini-DV Camcorder Sony DV Handycam A camcorder is a portable electronic device (generally a digital camera) for recording video images and audio onto a storage device. ...

Focus adjustment is mainly controlled electronically. 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. A datum is a statement accepted at face value (a given). Data is the plural of datum. ... Look up Electronic in Wiktionary, the free dictionary Electronic can refer to many things: Objects related to electronics The band Electronic. ... The word mechanical can mean one of several things: A device or principle described as mechanical relates to a mechanism or machine, or the realm of Newtonian mechanics. ... In engineering, electromechanics combines electromagnetism and mechanics. ...

An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust the lens directly.

Passive systems

Passive systems don't rely on any energy (in form of electromagnetic rays or soundwaves) to be sent from the camera to the subject. Instead, these systems utilize the light that is naturally reflected by the subject. Autofocusing can be achieved in two ways:

• phase detection
• contrast measurement

Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. This system is often utilized in SLR cameras. Contrast measurement is achieved by determining when maximum focus is adjusted to achieve the maximum contrast within the sensor field. This is a common method in video-cameras and digital-image cameras, and is measured through the actual optics in use. This way, passive autofocusing may not involve actual distance measuring at all. The single-lens reflex camera, more commonly known by the abbreviation SLR, uses a mirror placed between the lens and the film to project the image seen through the lens to a matte focusing screen. ... A SiPix digital camera next to a matchbox to show scale. ...

Pros and cons of the systems

Active systems will typically not focus through windows, since sound waves and IR light are reflected by the glass. With passive systems this will generally not be a problem, unless the window is stained.

Passive systems may not find focus when the contrast is low, notably on large single-coloured surfaces (walls, blue sky, etc) or in low-light conditions. In addition, passive systems are affected by subjects illumination. Active systems may focus correctly even in total darkness.