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.
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.