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Focal length
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<p>The focal length of an <a href="/facts/Optics/p0vq4S1T">optical</a> system is a measure of how strongly the system converges or diverges <a href="/facts/Light/jeofpMn4">light</a>; it is the <a href="/facts/Multiplicative_inverse/yczdYyCw">inverse</a> of the system's <a href="/facts/Optical_power/jmIWyX25">optical power</a>. A positive focal length indicates that a system <a href="/facts/Convergence_(optics)/NVQ2Xmcs">converges</a> light, while a negative focal length indicates that the system <a href="/facts/Divergence_(optics)/NVQ2Xmcs">diverges</a> light. A system with a shorter focal length bends the <a href="/facts/Ray_(optics)/PgUEC7Sm">rays</a> more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a <a href="/facts/Thin_lens/Gbt53F8z">thin lens</a> in air, a positive focal length is the distance over which initially <a href="/facts/Collimated_beam/lhd2hhPt">collimated</a> (parallel) rays are brought to a <a href="/facts/Focus_(optics)/QuRKfhg6">focus</a>, or alternatively a negative focal length indicates how far in front of the lens a <a href="/facts/Point_source/AHmNlt03">point source</a> must be located to form a collimated beam. For more general optical systems, the focal length has no intuitive meaning; it is simply the inverse of the system's optical power. </p><p>In most <a href="/facts/Photography/ezLPT43M">photography</a> and all <a href="/facts/Telescopy/BDwYahhR">telescopy</a>, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher <a href="/facts/Magnification/ACFaV1Bd">magnification</a> and a narrower <a href="/facts/Angle_of_view/QPjsVG1T">angle of view</a>; conversely, shorter focal length or higher optical power is associated with lower magnification and a wider angle of view. On the other hand, in applications such as <a href="/facts/Microscopy/GpHlElDS">microscopy</a> in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection. </p>