Autofocus

For the film of the same name see Auto Focus.

Autofocus (or AF) is a feature of some optical systems that allows them to obtain (and in some systems to also continuously maintain) correct focus on a subject, instead of requiring the operator to adjust focus manually.

General

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.

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.

History

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. 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.

Types

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.

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 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 utilized in film and digital SLR cameras. The system utilizes 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 range finder with a base identical to the lens' 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 sensor is typically a one-dimensional photosensitive strip (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1D, Nikon D2X) feature Area SIR sensors that are rectangular so as to provide two-dimensional intensity patterns in 2D. 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.

Contrast measurement 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. This is a common method in video cameras and consumer-level digital cameras that lack shutters. In this method, AF does not involve actual distance measurement at all.

Pros and cons of the systems

Active 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-coloured 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.

References

Norman Goldberg. Camera Technology : The Dark Side of the Lens
Sidney Ray. Applied Photographic Optics
Ralph Jacobson, Sidney Ray, Geoffrey G Attridge, Norman Axford. Manual of Photography: Photographic and Digital Imaging
J. M. Geusebroek, F. Cornelissen, A. W. M. Smeulders, and H. Geerts. Robust autofocusing in microscopy. Cytometry, 39(1):1-9, 2000.

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