Autofocus Systems

Autofocus systems rely on one or more sensors to determine correct focus

Some AF systems rely on a single sensor, while others use an array of sensors. Most modern SLR cameras use through-the-lens optical AF sensors, which also perform as light meters.

The speed and accuracy of through-the-lens optical autofocusing is now often more precise than what can be achieved manually with an ordinary viewfinder. (More precise manual focus can, of course, be achieved with special accessories such as focusing magnifiers.) Autofocus accuracy within 1/3 of the depth of field (DOF) at the widest aperture of the lens is not uncommon in professional AF SLR cameras.

Autofocus (AF) could also be called power-focus, as it often uses a computer to run a miniature motor that focuses the lens for you. Focusing is the moving of the lens in and out until the sharpest possible image of the subject is projected onto the film. Depending on the distance of the subject from the camera, the lens has to be a certain distance from the film to form a clear image. 

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Auto-focus systems can capture a subject in a variety of ways; here, the focus is on the person's image in the mirror.

Most multi-sensor AF cameras allow manual selection of the active sensor, and many offer automatic selection of the sensor using algorithms which attempt to discern the location of the subject. Some AF cameras are able to detect if the subject is moving towards or away from the camera, including speed and acceleration data, and keep focus on the subject -- a function used mainly in sports and other action photography.

The data collected from AF sensors is used to control an electromechanical system that adjusts the focus of the optical system. A variation of autofocus is called an electronic rangefinder, a system in which focus data are provided to the operator, but adjustment of the optical system is still performed manually.

The speed of the AF system is highly dependant on the maximum aperture offered by the lens. F-stops of around f/2 to f/2.8 are generally considered optimal in terms of focusing speed and accuracy. Faster lenses than this typically have very low depth of field, meaning that it takes longer to achieve correct focus, despite the increased amount of light. Most consumer camera systems will only autofocus reliably with lenses that have a maximum aperture of at least f/5.6, while professional models can often cope with lenses that have a maximum aperture of f/8, which is particularly useful for lenses used in conjunction with teleconverters.


The first mass-produced autofocus camera was the Konica C35 AF, a simple point and shoot model released in 1977. The Polaroid SX-70 was the first autofocus single-lens reflex camera, released in 1978. The Pentax ME-F, which used focus sensors in the camera body coupled with a motorized lens, became the first autofocus 35mm SLR in 1981. In 1983 Nikon released the F3AF, their first autofocus camera, which was based on a similar concept to the ME-F. The Minolta Maxxum 7000, released in 1985, was the first SLR with an integrated autofocus system, meaning both the AF sensors and the drive motor were housed in the camera body, as well as an integrated film advance winder - which was to become the standard configuration for SLR cameras from this manufacturer, as it would for Nikon. Canon, however, elected to develop their EOS system with motorized lenses instead.

Active autofocus

Active AF systems measure distance to the subject independently of the optical system, and subsequently adjust the optical system for correct focus.

There are various ways to measure distance, including ultrasonic sound waves and infrared light. In the first case, sound waves are emitted from the camera, and by measuring the delay in their reflection, distance to the subject is calculated. Polaroid cameras including the Spectra and SX-70 were known for successfully applying this system. In the latter case, infrared light is usually used to triangulate the distance to the subject. Compact cameras including the Nikon 35TiQD and 28TiQD, the Canon AF35M, and the Contax T2 and T3, as well as early video cameras, used this system.

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

Passive autofocus

Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. (However, an autofocus assist beam of usually infrared light is required when there is not enough light to take passive measurements.) Passive autofocusing can be achieved by phase detection or contrast measurement.

Phase detection

Phase Setection System Phase Setection System
Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. SIR TTL passive phase detection (secondary image registration, through the lens) is often used in film and digital SLR cameras. The system uses a beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Two optical prisms capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple rangefinder with a base identical to the lens's diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is in front focus or back focus position. This instantly gives the exact direction of focusing and amount of focus ring's movement. Although AF sensors are typically one-dimensional photosensitive strips (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1V, Canon EOS-1D, Nikon D2X) feature Area SIR sensors that are rectangular so as to provide two-dimensional intensity patterns. Cross-type (CT) focus points have a pair of sensors oriented at 90 to one another, although one sensor typically requires a larger aperture to operate than the other. Some cameras (Canon EOS-1V, Canon EOS-1D, Canon EOS 30D/40D) ) also have a few 'high precision' focus points with an additional set of prisms and sensors; they are only active with 'fast lenses' of certain focal ratio. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.

Contrast Detection

Contrast detection is achieved by measuring contrast within a sensor field, through the lens. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximum contrast is detected. In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light. As it does not use a separate sensor, however, contrast-detect autofocus can be more flexible (as it is implemented in software) and potentially more accurate. This is a common method in video cameras and consumer-level digital cameras that lack shutters and reflex mirrors. Some DSLRs (Olympus E-420, Panasonic L10, Nikon D90, Nikon D300 in Tripod Mode) use this method when focusing in their live-view modes. A new interchangeable-lens system, Micro Four Thirds, exclusively uses contrast measurement autofocus, and is said to offer performance comparable to phase detect systems.

Comparison of active and passive systems

High contrast bands can be projected onto low contrast subjects, seen here for Canon EOS 100Active systems will typically not focus through windows, since sound waves and infrared light are reflected by the glass. With passive systems this will generally not be a problem, unless the window is stained. Accuracy is often considerably less than passive systems.

Active systems may also fail to focus a subject that is very close to the camera (e.g., macro photography).

Passive systems may not find focus when the contrast is low, notably on large single-colored surfaces (walls, blue sky, etc.) or in low-light conditions. Passive systems are dependent on a certain degree of illumination to the subject (whether natural or otherwise), while active systems may focus correctly even in total darkness when necessary. Some external flash units have a special low-level illumination mode (usually orange/red light) which can be activated during auto-focus operation to allow the camera to focus.

Hybrid autofocus

In a hybrid autofocus system, focus is achieved by combining two or more methods, such as: Active and passive methods. Phase detection and contrast measurement. The double effort is typically used to speed up and improve AF function. In July, 2010, Fujifilm announced a compact camera, the F300EXR, which included a hybrid autofocus system consisting of both phase-detection and contrast-based elements. The sensors implementing the phase-detection AF in this camera are integrated into the camera's Super CCD EXR.[6] Currently it is used by Fujifilm FinePix Series, Ricoh, Nikon 1 series and Canon EOS 650D/Rebel T4i.


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