from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. The distance at which something (e.g. a lens, mirror etc.) is in focus; in air it is the distance along the optical axis from a "principal plane" of the lens to one of the lens's focal points.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. the distance from a lens to its focus


Sorry, no etymologies found.


  • But scientists came up with a new type of camera, using the folded-lens principle, where the light, instead of coming down a long, straight barrel, is bounced around a series of angled mirrored corners, which permits the focal length to be increased greatly without having to enlarge the camera itself.

    Ice Station Zebra

  • Then, ten years later, came what they called the Perkin-Elmer Roti satellite missile tracker, with a focal length of five hundred inches -- equivalent to a barrel-type camera forty feet long.

    Ice Station Zebra


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