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An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication. Autofocus methods are distinguished by their type as being either active, passive or hybrid variants.

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 sensors, with a separate sensor array providing light metering, although the latter can be programmed to prioritize its metering to the same area as one or more of the AF sensors.

Through-the-lens optical autofocusing is now often speedier and more precise than can be achieved manually with an ordinary viewfinder, although more precise manual focus can 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 common 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 whether 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; on Canon cameras this is known as AI servo, while on Nikon cameras it is known as "continuous focus".

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 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 dependent on the widest aperture offered by the lens at the current focal length. 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 (e.g.: f/1.4 or f/1.8) 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 widest aperture of at least f/5.6, while professional models can often cope with lenses that have a widest aperture of f/8, which is particularly useful for lenses used in conjunction with teleconverters.

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Agnostic Front is an American hardcore punk band from New York City. Founded in 1980, the band is considered an important influence on the New York hardcore scene, as well as for pioneering the crossover thrash genre.

The Abraham Initiatives is a non-profit organization based in Lod (Israel), New York City and London. Named after the common ancestor of both Jews and Arabs, Abraham. According to its website, it "strives to fulfill the promise of full and equal citizenship and complete equality of social and political rights for Israel’s Jewish and Arab citizens".

The Astraea Lesbian Foundation for Justice is an international charitable foundation based in the United States focused on issues related to LGBT and intersex rights for people of color. The organization provide grants to individuals and organizations, promotes philanthropy, and provides capacity building assistance.

American Forests is a 501(c)(3) non-profit conservation organization, established in 1875, and dedicated to protecting and restoring healthy forest ecosystems. The current headquarters are in Washington, D.C.

The Access Fund is a not-for-profit rock climbing advocacy group in the US. Their goals are twofold. First, keeping climbing areas open and gaining access to currently closed climbing areas. Second, they promote an ethic of responsible climbing and conservation of the climbing environment. The Access Fund was originally the access committee of the American Alpine Club and was created as the climbing community realized the need for an organization to represent climbing and climbers' rights in the US.

The Access Fund became its own 501(c)3 organization in 1991. The organization is based out of Boulder, Colorado and has a small full-time staff. Additionally, the Access Fund has a nationwide network of volunteers and local climbing organizations that work locally to keep climbing areas open and preserved for future climbers.

The Arthritis Foundation is a nonprofit organization that is dedicated to addressing the needs of people living with arthritis in the United States. There are more than 50 million adults and 300,000 children living with arthritis, the nation's leading cause of disability. The Foundation works to provide information and resources, access to care, advancements in science and community connections.

The Arthritis Foundation also publishes the magazine Arthritis Today. Through local offices nationwide, the Foundation sponsors a variety of year-round events to raise funds and create awareness, from local walks and runs to dinners, galas and other affairs.

The Arthritis Foundation has contributed to arthritis research since its founding in 1948, and has invested over $470 million toward medical research into the disease In addition, the organization advocates for changes to health care policies, legislation and government-funded research to improve the lives of individuals and families affected by arthritis.

The Foundation funds two patient registries, the Arthritis Internet Registry (AIR) and the Childhood Arthritis and Rheumatology Research Alliance (CARRA), to help control symptoms and accelerate the search for a cure by analyzing and correlating biological data.

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