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The focal length of a lens is the distance along the optical axis from the lens to the focus (or focal point). The inverse of a lens' focal length is called its power.
For a positive (converging) lens, the focal length is positive, and is the distance from the lens at which a collimated beam of light will be focused to a single spot. For a negative (diverging) lens, the focal length is negative, and is the distance in front of the lens to the point at which a collimated beam appears to be emerging from after passing through the lens.
For a thick lens (one which has a non-negligible thickness), or an imaging system consisting of several lenses (e.g., a photographic lens), two focal lengths can be defined: the front focal length (FFL) is the distance from the first (front) focal point of the system to the first optical surface. The back focal length (BFL) is the distance from the second (back) focal point to the last optical surface of the system. For a thin lens (one which has a negligible thickness), these two focal lengths are the same.
For a spherically curved mirror, the focal length is equal to half the radius of curvature of the mirror.
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