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focal length Dictionary Terms

cylindrical lens
A cylindrical lens is an optical component that has different curvatures along its two orthogonal axes, resulting in a shape...
nodal bench
A bench with instrumentation including a collimator, a microscope, positioners and a nodal slide used to rotate a lens about...
metalens
A metalens, short for "metasurface lens," is a type of optical lens that uses nanostructured materials to manipulate light...
standard lens
A lens whose focal length is roughly equal to the diagonal of the negative format of the camera on which it is mounted.
photographic field
The maximum angle of view that can be recorded by a camera. Field is a function of lens focal length and film format.
liquid mirror
A mirror composed of liquid, taking advantage of the parabolic shape of a spinning liquid and the fact that the mirror's...
variable-focus lens
A lens assembly containing several movable elements to permit changing of the effective focal length (EFL). Unlike a zoom...
mirror lens
An image-forming system having curved mirrors in place of transparent glass components. Mirror lenses are commonly used for...
MTF optimization
Computerized lens design algorithm that permits the lens MTF characteristic to be included and controlled during the...
lens speed
Also known as f number, lens speed is commonly represented as the ratio of the focal length to the diameter of the lens....
motion controller
In optics, a motion controller refers to a device or system that precisely controls the movement of optical components, such...
f number (f/#)
The expression denoting the ratio of the equivalent focal length of a lens to the diameter of its entrance pupil.
focometer
A device used to measure the focal length of an optical system or lens.
front focal length
relative aperture
The ratio of the diameter of the entrance pupil in an optical system to the equivalent focal length of that system. The...
telephoto ratio
In a telephoto lens, the ratio of the overall length to the focal length of the lens. It is generally about 0.8 to 0.9 in...
focal length
The focal length of a lens is the distance between the lens's optical center (or principal point) and the image sensor or...
telephoto lens
A compound lens so constructed that its overall length is equal to or less than its effective focal length.
equivalent focal length
 stereoscope
A small instrument containing a picture support and a pair of magnifying lenses so arranged that the left eye sees only the...
focal collimator
A collimator having, at one end of a tube, an objective lens, and at the other, a reticle with a pair of spaced lines...
simple magnifier
A short focal length (less than five inches) positive lens used to produce a magnified image of the object being viewed....
Ramsden eyepiece
An eyepiece consisting of two planoconvex lenses of the same focal length, with facing convex surfaces.
optical assembly
An optical assembly refers to a collection of optical components that are carefully arranged and aligned to perform a...
multifocal lens
A lens with internally adjustable elements to produce a range of focal lengths. Unlike a true zoom lens, a multifocal lens...
diopter movement
The adjustment of the eyepiece of an instrument to provide accommodation for the eyesight differences of individual...
infrared lens
An infrared lens is an optical component specifically designed and optimized for transmitting, focusing, or manipulating...
phototheodolite
A camera equipped with angular scales in altitude and azimuth. The scale readings are imprinted on the film when an exposure...
instantaneous field of view
Instantaneous field of view (iFOV) is a term commonly used in the context of remote sensing, imaging systems, and optical...
Huygenian eyepiece
An ocular having two planoconvex lenses that are formed from similar glass and that are separated by a space equal to half...
retroreflecting multipass cell
Two lenses, separated by a distance equal to the sum of their focal lengths, and retroreflecting mirror assemblies, one of...
iconometer
An instrument in which an object's image, produced by a lens of known focal length, is used to determine the object's...
optically compensating zoom system
A variable focal length lens system that retains the object in focus as one or more lenses move as a unit along its optical...
interchangeable lens
A lens that has a mount, usually bayonet or screw type in design, that can be used on a camera in place of lenses with the...
back focal length
The distance from the final optic within a system to the rear image point of the system. See focal length.
process lens
A lens that is symmetrical and designed to work between 1:1 and about 4:1. It covers a field of about ±20° at f/8...
digital camera
A digital camera is a device that captures and records still images or video in digital format. Unlike traditional film...
conjugate ratio
The ratio between the object distance and the image distance measured along the principal axis of a lens or mirror. An...
Fresnel number
In a lens, the square of the radius of its aperture divided by the product of the focal length and the wavelength. It...
Barlow lens
A negative lens used to increase the effective focal length of a telescope objective.
retrofocus lens
Also known as reversed telephoto lens. A lens consisting of an ordinary objective with a negative component near its front...
reduced focal length
The ratio of the first focal length of a lens to the refractive index of the medium containing the incident light; the ratio...
diopter
A unit of optical measurement that expresses the refractive power of a lens or prism. In a lens or lens system, it is the...
power
With respect to a lens, the reciprocal of its focal length. The term power, as applied to a telescope or microscope, often...
effective focal length
 machine vision lens
A machine vision lens is a specialized optical lens designed for use in machine vision systems, which are used for automated...
zoom lens
An optical system of variable focal length, the focal plane remaining in a fixed position. This result is achieved by moving...
pincushion distortion
An aberration of a lens system caused by an increase in lens focal length as the field angle increases. The amount of...
t number
The equivalent f number of a fictitious lens that has a circular opening and 100 percent transmittance, which would give the...
custom optic
A custom optic refers to an optical component that is designed, manufactured, and tailored to meet specific requirements or...
telephoto power
The ratio between the focal length of a lens having a longer focal length than that of the standard lens used with a camera,...
vertometer
A device that measures the back focal length or vertex power of a lens.
effective f number
For a lens with an obscured or noncircular aperture, the focal length divided by the effective aperture.
anamorphic system
An optical system with different focal lengths or magnification levels in perpendicular planes to the optical axis.
thin lens relationships
Formulas designating the relationships between image distance, object distance, focal length, refractive index, etc., of a...
Seidel aberrations
Seidel aberrations refer to a set of monochromatic aberrations in optical systems, named after the German mathematician and...
lens disc
A rotating disc that holds several lenses of differing focal length about a diameter. Used to switch lenses in a system...
millidiopter
A unit of metric measure equivalent to 1 thousandth (or 10-3 ) of the standard unit of optical power which is the diopter (1...
curvilinear distortion
A lens aberration in which the focal length varies radially outward from the center of the field. It has the effect of...
minimum object distance
Minimum object distance, often abbreviated as MOD, refers to the closest distance at which a camera lens or optical system...
image jump
In optics, the term image jump refers to a displacement or shift in the apparent position of an image when a change occurs...
bifocal lens
A two-part lens that has a different focal length for each part. Generally, it is used in eyeglasses to correct for both...
telephotography
1. A method of photographing distant objects with a lens of long focal length. 2. The reproduction of photographs over a...
absolute magnification
The value of the distance of distinct vision, minimum focusing distance or near point, divided by the focal length of the...
aperture ratio
The ratio of the lens aperture to its focal length (1/f/#).
nodal testing
The measurement of first- and higher order properties of a lens and its formed image, including effective focal length, back...
collimator
An optical instrument consisting of a well- corrected objective lens or mirror with a light source and or object/image (i.e....
Fresnel lens
A Fresnel lens is a type of optical lens that consists of a series of concentric grooves or steps carved into a flat, thin...
minimum spot size
The smallest linear diameter to which a laser or other beam of radiant energy is capable of being focused, depending on the...
wide-angle lens
A wide-angle lens is a type of camera lens that has a shorter focal length than a standard or normal lens, allowing it to...
afocal
An optical system with object and image points at infinity. Literally, "without a focal length."
axicon
An optical device that produces a line image lying along the axis from a point source of light; therefore, it has no...

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