Glossary

Our glossary contains the key technical terms in the field of optics and photography, from A for aperture to Z for zoom lens.

A

Aberration
When an image is created, an optical system reassembles object points in an associated image plane – a process whose precision can vary for reasons related to the laws of physics. Hence, a distinction is made between two main types of image distortion and their attendant subtypes: (1) monochromatic aberrations: spherical aberration, coma, astigmatism, and distortion; (2) chromatic aberrations: transverse and longitudinal chromatic aberration. In order to develop a successful lens design, lens glass and geometric parameters (radii, lens thickness, distances between lens elements and so on) must be combined in such a way that the lens provides a platform for images with the least possible amount of distortion.
Aspheric lens surface
Aspheric lens surfaces are optical surfaces that exhibit a mathematically defined, rotational symmetrical deviation from a perfect sphere. Aspheric lens surfaces serve a number of purposes, including correcting optical image distortion to a greater extent than would be achievable using either a large number of lenses or spherical surfaces on their own.
Astigmatism
A light beam striking a lens at an angle creates two different focal points. One of these focal points is at a right angle to the hypothetical sectional plane of the light, whereas the other is horizontal to the hypothetical sectional plane of the light beam. This state, which precludes the formation of exact pixels, is known as astigmatism and causes object details to vary in sharpness.
Aperture stop
Most photo lenses use an iris aperture, which allows for continuous adjustment of the aperture opening, is composed of a series of crescent shaped blades and allows for regulation of the intensity of the light that enters the lens. The aperture is also a key determining factor for depth of field. The unit of measurement for the aperture is the aperture number. A low aperture number such as 1.4 indicates a large iris-aperture opening and low depth of field, whereas a high aperture number such as 22 indicates a small opening and a large depth of field.
Aperture number
The ratio between the lens focal length and the entrance pupil. The entrance pupil is the diameter of the iris opening that is visible when you look into a camera lens. In lenses with fixed focal lengths, the aperture number is indicated in conjunction with the focal length, e.g. Digitar 2.8/28 mm: Digitar is the lens name, 2.8 is the highest aperture number, and 28 mm is the focal length. In many zoom lenses, the aperture number varies according to the focal length range, e.g. for Variogon lenses 3.5–4.8/35–350 mm.

B

Backlighting, against-the-light photograph
Backlighting a subject (e.g. if the sun is in the background of a landscape or a spotlight is behind the audience at a rock concert) can induce undesirable image effects, including poor contrast, artefacts, inadequate lighting resulting from use of a light meter and so on. As with dispersed light, quality lens coatings help mitigate the negative effects of backlighting.
Back focal length
The distance from the last surface of a lens to its image plane. See also focal length.
Brightness falloff
Unlike vignetting, brightness falloff, which is most frequently observed in wide-angle photos, reduces the intensity of light beams that enter the imaging system from an acute angle, causing the edges of the image be much darker than the centre.

C

Chromatic aberration
Any colour-related image distortion induced by the dispersion characteristics of lens glass (also see “Transverse chromatic aberration” and “Longitudinal chromatic aberration”).
Coating
See multicoating.
Colour fringe
A non-corrected colour distortion that occurs at non-radial, highly contrasting edges.
Coma
Also referred to as asymmetry distortion, coma is induced by the presence of object points adjacent to the lens axis that enter the lens as wide open bundles, which induce artefacts in the image capture system in lieu of a sharp pixel.
Contrast
The difference in the brightness of two areas of differing brightness, based on the brightness of one such area. In the modulation transfer function (MTF), the relationship between contrast and resolution is defined. MTF is a key criterion for assessing the optical imaging quality of a lens.

D

Distortion
A geometric imaging aberration that is measured for various aperture angles using various imaging metrics. A distinction is made between barrel-shaped and billowing distortion, depending on whether the enlargement decreases or increases the size of the image field edges.
Dispersion
Dispersion in connection with plastic or glass lens materials refers to a phenomenon whereby light is split into its constituent spectral colours. From the standpoint of physics, dispersion is the correlation between the dispersion value and light wavelength. A well known example of light dispersion is constituted by the rainbow colours you see with the aid of a prism, whose red light exhibits a shorter wavelength than the prism’s blue light. Some lens materials have the capacity to fan out light to one degree or another. The use of specific combinations of glass allows lenses to be designed that minimise colour distortion (also see “ED glass”).

E

ED glass
Low-dispersion lens glass, i.e. glass with a weaker correlation between the glass dispersion number and light wavelength. ED glass is primarily used in specific types of lenses, with a view to reducing colour distortion for the lens as a whole.

F

Field curvature
A phenomenon whereby the points on the flat surface of an object are in sharp focus and at the same time curved in the imaging system. In uncorrected imaging systems, this curvature can result in phenomena such as an object that needs to be in sharp focus going increasingly out of focus on ground glass from the centre to the edge of the image.
Flare
Diffuse light inside a lens or camera housing that may be induced by any number of factors. Light dispersion, which bleaches out the image and reduces image contrast, can be attenuated in various ways, including placing lens components with a black-matte painted coating in the lens light path, or the use of optimally designed coating layers.

I

Image sensor
An electronic, photosensitive component that is composed of millions of photodiodes and that is used as an image receiver in digital cameras. This photoelectronic image sensor converts the light that passes through the lens into per-photodiode charges of varying sizes that are then processed by the camera’s electronics and converted into image information by the camera software. The best-known types of image sensors are CCD and CMOS sensors.

L

Longitudinal chromatic aberration
A dispersion-induced phenomenon whereby various-coloured objects are visualised at varying distances.

M

Multicoating
Anti-reflective lens surface coating chiefly composed of a series of dielectric layers and exhibiting low and high refraction substrate properties. Lens coatings are also referred to as multicoating layers. The effect exerted by lens coatings is based on the interference effect, which attenuates reflection in whole or in part. Specific effects can be achieved by strategically varying layer thickness and count. Lens coatings are applied via steaming in high-vacuum ovens.
MTF, Modulation transfer function
Quantitative characteristics of the imaging performance of a lens, in light of all types of imaging distortion exhibited by the lens. To assess this function, increasingly narrower pairs of black and white lines whose contrast is known are formed using the lens, whereupon the changes in the image contrast are measured. The ratio between the image and object contrast indicates the modulation factor for each line frequency. The modulation transfer function (MTF), which is also referred to as the contrast transfer function, is determined by plotting the modulation factor onto the number of lines.

R

Resolution capability, resolving power
The capacity of an imaging lens to reproduce two neighbouring object points as details that can be clearly distinguished from each other. Resolution usually connotes the minimum angular distance that must be maintained between two object points in an imaging lens system in order for these two points to be reproduced as two separate points in the imaging system.

S

Sharpness
An assessment criterion for the detail resolution of an image. Sharpness assessments are based on the number of resolved black and white line pairs, edge sharpness at contrast transitions, subjective perception of sharpness, and numerous other factors such as shooting conditions, subject, camera technology and so on.
Sperical aberration
A phenomenon whereby paraxial light beams strike the lens at differing heights. These light beams intersect the lens axis at various points, resulting in rotationally symmetric dispersion artefacts around the paraxial pixels.
Symmetry
The requirement that image quality be rotationally symmetrical means that image quality must meet the required quality criteria in all four corners of the image. Image aberrations can be provoked by anomalies such as a raked lens element in a camera lens or the image sensor being mounted in the wrong place.

T

Transmission
The transmission of light through one or more lens elements. The spectral makeup of the light passing through the lens elements is modified in accordance with the type of glass used. For example, lens elements that filter out ultraviolet light remove the ultraviolet component from the light but allow all their components to pass through the lens element intact.
Transverse chromatic aberration
A dispersion-induced phenomenon whereby various-sized coloured images of an object are visualised in an imaging system.

V

Vignetting
Mechanically induced white shading on the edges of a picture. Unlike white shading, vignetting is induced by lens fitting edges.
Variogon lens
Variogon is a brand name for Jos. Schneider Optische Werke GmbH zoom lenses, which meet defined quality standards for optimal reproduction performance. This classic brand has been well established on the market for many decades. Already in the mid 1950s, Schneider was developing and manufacturing lenses for movie cameras. The outstanding precision and performance of these Vario lenses ultimately paved the way for their use in TV cameras, surveillance cameras, the small and medium format sector, enlargers, still picture projection, OEM applications and modern digital cameras.

Z

Zoom lens
A lens with a continuously adjustable focal length (also referred to as a short zoom or Vario lens), whereby image sharpness is retained across the entire focal length range. In a zoom lens, focal length is varied via precise shifting of lens groups in relation to each other, thus ensuring that optical image quality is obtained across the entire focal length range. A special type of glass known as ED glass is often used in zoom lenses, as is aspheric glass.