With respect to a lens or mirror, the distance from the lens or mirror at which a parallel beam of light rays will be focused to the smallest size possible for the lens or mirror. The focal length of a simple converging (convex) lens can be measured by focusing the rays from the sun to the smallest point possible and measuring the distance from the image to the lens.
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(2009). Focal Length. In: Li, S.Z., Jain, A. (eds) Encyclopedia of Biometrics. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-73003-5_517
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DOI: https://doi.org/10.1007/978-0-387-73003-5_517
Publisher Name: Springer, Boston, MA
Print ISBN: 978-0-387-73002-8
Online ISBN: 978-0-387-73003-5
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