Longman Dictionary of Contemporary English
Wiktionary
n. (context optics English) 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.
WordNet
n. the distance from a lens to its focus [syn: focal distance]
Wikipedia
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more sharply, bringing them to a focus in a shorter distance.
In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with a wider angle of view. On the other hand, in applications such as microscopy 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.
Usage examples of "focal length".
We don't know the focal length, it's never been revealed: we do know that, given the right 'atmospheric conditions, a white saucer on dark surface will show up clearly from three hundred miles up in space.
The engineer swung the laser toward the attacker, readjusting the focal length with his senses, fighting against his own headache and the knives in his eyes.
As recently as fifteen years ago the maximum focal length of any camera outside an observatory was about fifty inches.
If we could warp spacetime hard enough to create a short focal length lens that was kilometers in aperture diameter, we could see freckles on an ant's ass from the moon.
His left hand made minute adjustments to the focal length of his lens, shortening it to keep as nearly as possible the whole drifting mass within his field of view.
The increase of power also changed the focal length of both lenses so that from time to time it was necessary to reposition them.
The focal length of the telescope was almost infinitely adjustable: in this case Van Gelder would probably use a one in a hundred resolution, which would have the optical effect of bringing the intruder -- if intruder it was -- to an apparent altitude of four hundred feet.
Being made of metal made for steadiness, at least, as the split hair proved, and the auxiliary lenses changed the focal length of his eyes to his optional telescopic or microscopic sight.
Due to some equipment malfunction, missed at the time, the camera's focal length had gone off slightly and the face was blurry—.
Due to some equipment malfunction, missed at the time, the camera's focal length had gone off slightly and the face was blurryit looked like one of the flawed experiments from the early days of photography.
Due to some equipment malfunction, missed at the time, the camera's focal length had gone off slightly and the face was blurry-it looked like one of the flawed experiments from the early days of photography.
It focuses light without lenses, and it can be adjusted to gather light over a wider and wider area without any change in focal length.