American Heritage® Dictionary of the English Language, Fourth Edition
- n. The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus.
- n. optics 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.
- n. the distance from a lens to its focus
“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.”
“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.”
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