# focal length

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## focal length

, distance
the distance from the focal point of a lens or mirror to the reflecting surface of the mirror or the centre point of the lens

## Focal length

A measure of the collecting or diverging power of a lens or an optical system. Focal length, usually designated f in formulas, is measured by the distance of the focal point (the point where the image of a parallel entering bundle of light rays is formed) from the lens, or more exactly by the distance from the principal point to the focal point. See Geometrical optics

The power of a lens system is equal to n/f, where n is the refractive index in the image space (n is usually equal to unity). A lens of zero power is said to be afocal. Telescopes are afocal lens systems. See Diopter, Lens (optics), Telescope

Focal length and focal plane

## focal length

Symbol: f . The distance from the center of a reflecting surface or refracting medium to the focal point or focus (see illustration). With a converging system, such as a paraboloid surface, a concave mirror or thin convex lens, the focus, F, is the point to which a narrow beam of light, radio waves, etc., from a distant object, i.e. a parallel beam closely aligned to the axis, is brought to a sharply defined or focused image. In a convex mirror or thin concave lens it is the point from which a parallel beam, made divergent by the mirror or lens, appears to diverge. If the two surfaces of a lens do not have identical curvatures the lens will have two different focal lengths and focal points, depending on which surface the light falls.

The focal plane is the plane through the focus, at right angles to the optical axis, in which the image of a distant object will be formed. In some cases, as in the Schmidt telescope, the image is focused on a curved surface – the focal surface – rather than a plane. See also effective focal length.

## focal length

[′fō·kəl ‚leŋkth]
(optics)
The distance from the focal point of a lens or curved mirror to the principal point; for a thin lens it is approximately the distance from the focal point to the lens. Also known as focal distance.

## focal length

The linear measurement along the optical axis between the center of the film and the center of the lens, with the camera focused at infinity.

## focal length

The height and width of the scene captured by a camera lens, which is known as the "field of view." In the past, lenses with fixed focal lengths were commonly used, which means every picture was captured with the same field of view. Today, most lenses are "zoom lenses" with a variable range of focal lengths and a variable field of view. See f-stop.

Wide Angle to Telephoto These two pictures were taken at both ends of a Nikon 18-70mm zoom lens on a digital SLR. At 18mm (top), the field of view is the widest. At 70mm (bottom), the scene is the narrowest. The 18-70mm on the Nikon camera is equivalent to a 27-105mm lens on a 35mm analog SLR camera (see crop factor).

Wide Angle to Telephoto These two pictures were taken at both ends of a Nikon 18-70mm zoom lens on a digital SLR. At 18mm (top), the field of view is the widest. At 70mm (bottom), the scene is the narrowest. The 18-70mm on the Nikon camera is equivalent to a 27-105mm lens on a 35mm analog SLR camera (see crop factor).
References in periodicals archive ?
There were a lot of spectacular views but could only be appreciated with a long focal length.
Looking for more than one wide-angle focal length to cover your total field of view?
The mold temperature is the most important processing factor for focal length of molded microlens by using PMMA or PC material.
Cameras with interchangeable lenses typically come with a zoom lens covering a moderate range, such as 28mm to 100mm to focal lengths.
This combination of advanced Canon optical technologies also minimizes curvature of field, chromatic aberration, chromatic aberration of magnification, and axial chromatic aberration throughout the entire field regardless of focal length or subject distance, stably enabling peak optical performance.
8 (the optically stabilized versions) from both Canon and Nikon are also often used to shoot portraits thanks to their long focal lengths and wide aperture which offer the ultimate in subject isolation.
Ultra wide-angles, long telephotos and macro lenses still require a single focal length construction to take full advantage of the maximum aperture available, thereby allowing as much light to be resolved as possible.
The C22X23 reaches a focal length of 500 mm with a range of 23 mm to 506 mm.
As the water climbed the walls under various voltages, the curvature of its upper surface changed, modifying the lens' focal length.
That means that the focal length of a practical lens can be no less than about 40% of its diameter.
Three glass mold aspherical lenses provide excellent correction for distortion, color aberration and field curvature, and an inner focusing system corrects the fluctuation of aberration to maintain image quality regardless of the focal length.
In SLR systems, when extremely large image angles are needed, the distance between the rear element and the image plane must be considerably longer than the focal length -- the original field of application of Distagon lenses.

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