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A3

A standard size for a sheet of paper or card in the ISO system having an aspect ratio of 1:sqrt 2 (1: 1.4142) - A3 sheets measure 297 x 420 mm (11.7" x 16.54").

A4

A standard size for a sheet of paper or card in the ISO system having an aspect ratio of 1:sqrt 2 (1: 1.4142) - A4 sheets measure 210 x 297 mm (8.27" x 11.7").

A5

A standard size for a sheet of paper or card in the ISO system having an aspect ratio of 1:sqrt 2 (1: 1.4142) - A5 sheets measure 148 x 210 mm (5.83" x 8.27").

Aberration

The inability of a lens to produce a sharp image, particularly at the edge of the field of view. There are six principal aberration types - distortion, astigmatism, spherical, chromatic, coma and field curvature.

Absolute temperature

Often referred to as absolute zero, or -273.16 degrees C. The physical temperature at which all molecular movement is generally considered to cease. This temperature is the reference point of the Kelvin (degrees K) scale. It is used in photography in the measurement the "colour temperature" of light.

Accessory shoe

A fitting on the top of a camera, or other item of photographic equipment, which supports and sometimes provides electrical connections to accessories such as flash guns, viewfinders and rangefinders.

Achromatic colour

Neutral colours such as white, black and all the greys that have no hue.

Achromatic lens

A lens that is constructed using different types of glass to minimise chromatic aberration.

ADC (analogue to digital conversion)

The process of changing continuously varying analogue data into a series of representative digitized values.

Aerial perspective

The impression of depth given to an image by the presence of haze. Haze increases extraneous ultra-violet light, producing an overall increase in image density that obscures detail and a tonal gradient between foreground and horizon.

Aerial survey

Surveys conducted from an aircraft with the general objective of gathering information and capturing images systematically from a large area. Data is normally collated and interpreted in some way before being presented in a report.

Ageing

A physical change in material which takes place over a period of time, such as the fading of colours. The rate of change in photographic media is often related to exposure to light, humidity and high temperatures.

AGL

Above ground level.

Air-to-air photography

Photography of an aircraft in flight, from another aircraft in flight.

Air-to-sea photography

Photography of a ship or other sea-going craft undertaken from the air.

Airbrushing

A traditional and specialised method of retouching photographs to remove or conceal unwanted areas or elements. This was traditionally done using a fine spray gun, but is now largely untertaken using digital image manipulation software.

Algorithm

A set of rules defining a logical or mathematical operation.

Aliasing

The visible result of using as part of the digitization process an input sampling rate too slow to preserve image detail, otherwise known as jaggies. The sampling rate is less than twice that of the spatial frequency.

Ambient light

Existing light over which there is no direct control, as opposed to artificial sources, in a particular environment.

Angle of incidence

The angle subtended by a light ray falling on a reflecting surface and a line perpendicular to the reflecting surface. This angle is equal to the angle of reflection.

Angle of reflection

The angle subtended by a reflected light ray and a line perpendicular to the reflecting surface. This angle is equal to the angle of incidence.

Angle of refraction

The angle between a refracted light ray and the normal to the surface.

Angle of view

The angular extent (measured in degrees) of a scene that can be captured by a particular lens. The focal length of a lens determines its angle of view.

Angstrom unit

Unit, equal to 10-10 metres, used to measure the wavelength of light.

Anti-aliasing

Removal or reduction of the jagged or pixelated edges sometimes seen in a digital image. The effect is achieved by averaging the colours of adjacent pixels to create a smoother colour transitions.

Aperture

The physical size, calibrated as f/stops, of the near-circular hole in a lens diaphragm through which light passes.

Aperture priority

An autoexposure mode that allows the user to select manually a particular aperture and leaves the camera to select a corresponding shutter speed.

Aperture stop

A doubling or halving of the amount of light passing through a lens achieved by changing the physical diameter of a diaphragm aperture.

Apochromatic lens

A lens corrected as far as possible for chromatic aberrations in each of the three primary colours.

Application (App)

A computer program such as a text or image processor, internet browser or database.

Archival

The enhanced capability of some media, such as some DVDs and certain printing papers, to retain data or images over a period of many years.

Aspect ratio

The ratio of width to height of a rectangular area such as a film or sensor format, or a printed image.

Aspherical lens

A lens, or lens element, having surface geometry that is other than spherical. Such lenses tend to be flattened around the edges to reduce aberrations, and consequently produce higher-quality images.

Attachment

A digital file attached to an e-mail and delivered at the same time as the message. Low-resolution images may be transmitted in this way.

Autoexposure

Automated exposure control removing the need for manual adjustment of some or all exposure-setting controls.

Autofocus

Automated focusing system based on placing a marked target area on a subject. An electronic or programmed system in the camera then sets the optimum distance setting to ensure the best image clarity.

Average scene brightness

A reflectance of 18% is considered to represent average scene brightness. Exposure meters are typically calibrated to provide correct exposure for a subject of average brightness, and may consequently produce erroneous readings when measuring subjects with higher or lower average values - such as snow scenes.

B (Bulb) setting

This function keeps a shutter open as long as the release button remains depressed.

Back-up

A send copy of computer files, or the process of making them, for the purpose of protecting against their loss or damage.

Background paper

Rolls of paper used in a studio to provide plain-coloured backgrounds. The rolls are normally suspended from ceiling mountings or from a pair of adjustable tripod stands equipped with a crossbar. The paper is available in a range of standard widths (typically 2.7 metres) and a large range of colours.

Banding

An undesirable artefact of colour gradation produced in computer imaging systems when smooth colour gradients are rendered as blocks of a single colour.

Barn doors

An accessory used with studio light sources to control the direction and spread of the light.

Barrel distortion

A lens aberration that distorts the shape of images. Magnification increases radially inwards, so the straight edges of a square object bow outwards.

Bayonet

Modern DSLR cameras, and 35mm SLR film cameras, use a bayonet lens fitting which allows a lens to be attached or detatched using a 90-degree turn clockwise or anti-clockwise. Camera manufacturers such as Nikon, Canon, Pentax, Olympus and Sony have their own specific bayonet design so that, for example, a Pentax lens cannot be fitted to a Nikon camera without purchasing a custom-made adaptor.

Bit-depth

The number of binary digits assigned to each sample in a sequence of data used to represent an image. 8-bit information registers one of a possible 256 values (colours or densities), whereas 16-bit data registers one of 65,536 values.

Bleed

Information or material that overflows the edge or trim-line of a printed image.

Blu-Ray

A high-capacity optical disk storage system designed to supersede the DVD format. It uses blue laser technology at a wavelength of 405 nanometres to store up to 50 Gigabytes (GB) of data on a single disk.

Bokek

The aesthetic quality of out-of-focus elements of an image. Good bokeh is achieved when such areas appear soft and well merged. The term originates from the Japanese language.

Boot

To start or restart a computer by loading the basic operating system.

Bounced flash

The diffusion of light from a flashgun by directing it at a suitable reflecting surface such as a white ceiling.

Bracketing

A process of making several shots of the same subject, using incremental changes of aperture or shutter speed, to overcome exposure uncertainties.

Brightness range

The range of tonal variation in an image or scene, usually expressed as a ratio or in f/stops.

Brolly

An umbrella (brolly) constructed from flexible metal ribs and white or silver reflective fabric that is used to bounce and hence diffuse the light produced by studio flash heads.

Bromoil

An old printing process, patented in the 1850s by Alphonse Poitevin, in which greasy ink on gelatine sensitised by potassium dichromate is used to produce an image.

Buffer

A memory in an output device, such as a digital camera or printer, which stores data temporarily and feeds it to the device at an appropriate rate.

Burning-in

A darkroom or equivalent digital technique for increasing local contrast and density in an image.

Byte

A unit of digital information comprising a number of binary states or bits. Typically, one byte consists of 8 bits. 16-bit values therefore consist of 2 bytes.

C-41

The chemistry used to process most colour negative films.

Cable release

A flexible cable used to release a camera shutter without causing movement and consequent blurring of an image.  It is normally used in conjunction with a tripod and a slow shutter speed.

Cache

RAM used to hold computer data recently read from a mass storage device. Its purpose is to speed up processing.

Camera format

The size and aspect ratio of an image produced on film or a digital sensor by a camera body. The format is determined by the physical dimensions of the focal plane or image sensor.

Camera movement

Movement of a camera during the exposure period, perhaps causing loss of detail or blurring.

Camera obscura

The camera obscura consists of a darkened room with a small aperture opened in one wall. An inverted image of the scene outside the room is projected through the aperture and onto the opposite (usually white) wall. Eventually a converging lens was fitted in to the aperture to produce clearer and sharper images.

Capacity

The quantity of computer data that can be held by a storage device, usually quoted in MB or GB or TB.

Catadioptric lens

A long focal length lens using mirrors within its construction. The mirrors allow a long focal length to be achieved within a comparatively short lens barrel. This type of lens is also known as a mirror or reflex lens.

Catch-light

Small specular reflections of a light source usually seen in the eyes in portraiture. Lack of catch lights in portraits may be regarded as a deficiency.

CCD

Silicon chip used as an electronic sensor to replace film in a digital camera. It has electrodes arranged in a rectangular array. The electrodes a light sensitive and correspond to pixels. The light level measured by each electrode is turned into a digital code.

CD-R

Compact disk used as a read-only memory (CD-ROM). Data can be written onto it only once.

CD-RW

A rewritable compact disk onto which data can be written any number of times.

Centre-weighted metering

Exposure determination system commonly found in SLRs and DSLRs giving greater emphasis to brightness in the centre of the field of view.

Changing bag

A light-proof black fabric bag used to handle film and other light-sensitive materials in daylight. Most have two separate layers equipped with zips, and a pair of sleeves with elastic armholes.

Chroma

The purity of a colour measured as a product of hue and brightness, or the degree of departure of a colour from the neutral. Colours having a low chroma value are usually described as weak, whereas those with a high chroma are said to be strong or highly saturated.

Chromogenic films

B&W films that can be processed using standard C-41 colour chemistry to produce monochrome prints.

Circle of confusion

The diameter of a spot that cannot be distinguished from a point in an image at a normal viewing distance of 250mm. This measurement is an essential element in calculating depth of field.

Cloning

Copying one part of an image directly to another area.

CMYK

Cyan, magenta, yellow and black colour model.

Colorimeter

An instrument for measuring colour, and used to calibrate a computer monitor.

Colour balance

An adjustment made in a photographic process to ensure that neutral greys in a subject remain neutral in an image.

Colour cast

A false predominance or bias of a particular colour spread evenly throughout an image, and usually most apparent in light neutral tones.

Colour gamut

The range of colours a particular device or reproduction system is able to produce.

Colour management

The process of controlling the colours in an image reproduction system such that they are repeatable and acceptably accurate. Key components of colour management are monitor and printer calibration.

Colour rendering index (CRI)

A quantitative measure of the ability of a light source to reproduce colours - measured against their perceived appearance in natural light or under a reference illuminant. natural sunlight is considered to have a CRI of 100.

Colour space

A defined range of colours producible by an output device.

Colour temperature

Measure of the colour of a light source in terms of physical temperature (degrees Kelvin) of a black body radiator. Daylight approximates to 5,400oK and a sunset to 2,800oK.

Coma

The inability of a lens to render off-axis point sources of light as circular. Points appear as comet-shaped blurs, from which the name coma is derived, with the tails flaring toward the centre of an image. The problem is minimized by closing down the lens diaphragm.The aberration is difficult to eliminate in wide-angle lenses with large maximum apertures.

CompactFlash card

A memory card designed to store images within a digital camera. Many digital cameras with PC card interfaces use this storage technology which conforms to a standard supported by the CompactFlash Association. CompactFlash is ATA compatible and the cards fit Type II or Type III slots when used with a passive adapter.

Complementary colours

Pairs of colours of light which, when combined in equal proportions, produce white light - ie cyan and red, yellow and blue.

Compression

The process of encoding digital files in a space-efficient manner. Compression algorithms remove unnecessary information from files in a way that allows it later to be replaced. No compression of physical storage is involved.

Contact sheet

A same-size print of all the negatives on a roll of film used to select images for enlargement.

Contrast

The brightness range in a scene, or the difference in brightness between adjacent areas of tone. Also, the rate at which processed film density increases with greater exposure.

Convergence

The appearance of parallel lines in a view, as seen in an image, when photographed from an angle.

CPU (Central Processing Unit)

An electronic component that controls the principal functions of a computer or camera. Automatic cameras have at least one CPU to control essential functions. More sophisticated models may have numerous CPUs to handle exposure, autofocus etc. Some autofocus lenses also have built-in CPUs to communicate information such as focal length, focus distance, and lens type to the body of the camera.

Crop

To use or remove part of an image typically to improve composition or make it fit a given space.

Cropped sensor

A sensor in a digital camera having dimensions smaller than those of the comparable film-based format.

Cut and paste

To remove a selected part of an image, or other digital file, and store it temporarily in a memory before reusing in, or pasting it to, another location.

D-SLR

A single-lens reflex camera that uses a digital sensor rather than film.

DAC (Digital to Analogue Conversion)

The process of changing a series of representative digitized values into continuously varying analogue data.

Dark current noise

Dark current noise is caused by the accumulation of stray electrons in sensor pixels not exposed to light. It reveals itself as random noise in shadow or dark areas of images. The phenomenon is cumulative over time and so becomes more noticeable when exposure periods are very long.

Definition

An assessment of the clarity of an image, or of the amount of detail which can be perceived within it.

Depth of field

The range of distances from a lens within which an object is considered acceptably sharp.

Depth of focus

The range of distances, measured along the optical axis, through which the image of an object (on the film or sensor side of a lens) may be moved whilst retaining acceptable sharpness (as defined by the diameter of the acceptable circle of blur).

Depth oif field preview

A control that shuts lens aperture down to a pre-selected level and makes depth of field visible through the viewfinder.

Diaphragm

A set of crescent-shaped blades forming a near-circular hole at the centre of the lens, and used to control the amount of light reaching a film or sensor. The diameter of the hole varies as the blades slide over each other under the control of an external ring. The aperture can typically be varied from a tiny opening a few millimetres in diameter to the maximum diameter of the lens.

Differential focus

Control of shallow depth of field envelope to focus sharply upon a particular part of an image, allowing other areas to drop out of focus.

Digital negative

This is an open format file format similar to a RAW file that is consequently independent of manufacturer-specific software. Digital negatives require processing before they can be viewed or processed, but are capable of delivering more colour and dynamic range than TIFF or JPEG files.

Dispersion

Dispersion is a phenomena which separates spatially the various wavelengths (colours) of which white light is comprised. A dispersive material has a different refractive index for each wavelength of light, and consequently refracts (deflects) each wavelength by a different angle. The simplest demonstration of dispersion is the rainbow of colours produced by passing a ray of white light through a prism.

Dodging

A darkroom or equivalent digital technique for decreasing local contrast and density in an image.

Download

To transfer computer files to a more local or less significant location ie from a personal computer to a CD.

DPI (Dots per inch)

A unit used to measure the resolution of an image produced by an output device such as a printer. The greater the number of dots per inch the higher the resolution of the image.

Duotone

A mode of printing an image using two colours.

DVD

Digital versatile disk; similar to a CD but offering larger storage capacity.

DX-coding

A system that reads bar codes on film cassettes and uses the information to set ISO rating automatically.

Dynamic range

The range of energy levels that can be captured by a recording or reproduction device. A good digital camera might manage 10 stops and a film scanner perhaps 8 stops.

E-6

The chemistry used to process most slide films (excluding Kodachrome).

Effective pixels

The number of pixels used to make up an image. A typical sensor uses some pixels to measure background electron flow as a way of reducing noise. Total pixels therefore typically exceed effective pixels.

Electromagnetic spectrum

The range of frequencies of electromagnetic radiation. It extends from radio waves to gamma rays and incorporates the spectrum of visible radiation (light) to which the human eye is sensitive.

Electronic viewfinder

Typically a small LCD monitor used to display an image captured by a sensor in a digital camera.

Emulsion

Light sensitive silver halide crystals, combined with other chemicals, laid on a film base or other backing material.

Exposure

Process or period during which light is allowed to fall on a sensor or sensitized material to produce a latent image. Exposure is usually controlled by opening a shutter for a specific period of time.

Exposure compensation

Generally, the process of manually intervening in, or offsetting by small increments, an exposure determination made by an automatic exposure setting system.

Exposure latitude

The range of exposure variation that a film, or electronic sensor, can tolerate without loss of image quality.

Extension tube

A hollow tube inserted between a camera body and a lens to increase magnification. It is useful for close-up work.

f/number

The setting of a diaphragm aperture controlling the amount of light passing through a lens. Each f/stop is the number by which the focal length of a lens must be divided to give a particular aperture diameter.

Fast lens

A lens with a large maximum aperture for a particular focal length. A 50mm f/1.4 lens is considered fast, as is a 400mm f/2.8 lens.

Field of view

The extent of a subject visible with the eye, a particular lens or other optical instrument. In the case of a lens this is typically quoted as the angle subtended at the image plane by the limits of the field.

File format

The structure, organisation and codes used by a computer program to store data.

Fill

Light from a flash or reflector used to illuminate shadows and dark areas, and hence reduce contrast in an image to within the exposure latitude of a film or sensor.

Film plane

The plane, at the back of a camera, in which a film lays and upon which focus is set.

Film speed

The rate at which a light-sensitive material responds to light falling upon it. ISO film speeds are the internationally accepted norm.

Filter factor

A measure of the increase in exposure needed to compensate for the presence of a filter placed over a lens. A factor of 3x indicates that a tripling of exposure is required.

First-generation image

An analogue image created by the direct application of light from a subject falling upon a light-sensitive material, eg images created on transparency or negative film in a camera. The term is not normally applied to digital images.

Fish-eye lens

A lens having a very wide-angle of view characterized by severe barrel distortion and perhaps producing a circular image.

Flag

A piece of card or polystyrene used to block and hence limit the spread of artificial light sources in a studio.

Flare

Non-image-forming light internally reflected within a lens which reduces colour saturation and contrast, and may produce irregular smudges of colour across an image.

Flash compensation

Generally, the process of manually intervening in, or offsetting by small increments, a flash exposure determination made by an automatic exposure setting system.

Focal length

In general, the distance between the optical centre of a lens and an in-focus image of a subject at infinity projected by it.

Focal-plane shutter

A type of shutter used universally in 35mm SLR and DSLR cameras. The mechanism is positioned behind the lens and a little in front of the focal plane. It consists basically of two fabric or metal blinds. One blind slides across the film or sensor to open the shutter and the second follows to close the exposure once the exposure period has elapsed.

Full-frame sensor

A sensor in a DSLR which has the physical dimensions of a 35mm frame - ie 36mm x 24mm.

Gels

Sheets of coloured transparent media used to add various colours to light sources.

Ghost images

Bright spots of light, normally taking the form of the lens diaphragm, which appear in a camera viewfinder or in a recorded image when a lens is directed close to an intense source of light. Multi-layer coating of filters and lens elements helps reduce the effect, but cannot completely eliminate it. The effect is not always undesirable.

Gigabyte (GB)

One thousand million (one billion) bytes of computer data - equivalent to 1,000 Megabytes.

Gobo

A cardboard or metal shape used with a suitable light source to project a controlled shadow onto a subject. Gobo are normally used inside a focusing spot lights.

Graduated filter

A filter whose density of colour changes smoothly either from maximum to minimum or from one colour to another.

Grain

The appearance of a single light-sensitive crystal or individual dye cloud in a processed image.

Grey card

If all the tones in a typical image are mixed together, the result would be 18 per cent grey. A standard grey card reflects 18 per cent of the light falling upon it, and is therefore regarded as a standard against which exposures can be measured, and reflected-light exposure meters calibrated.

Greyscale

A set of defined brightness steps used by a system. An 8-bit scanner digitizes to a 256-step scale from 0 (black) to 255 (white).

Guide number

A number used in flash photography to obtain a correct lens aperture for subject distance and ISO rating. Guide number for a particular ISO rating, divided by metric distance, gives lens aperture.

Halation

The phenomenon of halos around highlights in an image. The effect is produced when light is reflected back from the back of the film-base. Film bases are consequently given a light-absorbing coat known as an anti-halation back to reduce or prevent the effect.

High-key image

An image consisting almost exclusively of light tones.

Highlight

The brightest part of an image, particularly when specular or causing notable over-exposure.

Histogram

In digital photography, a statistical representation of the number of pixels of a particular value plotted against the range of possible values.

Hot-shoe

A reasonably standardized connector found on the top of a camera designed to link with electronic flashguns.

Hue

The perceived colour of light, for example red or blue. Hue varies with wavelength. Colour is characterized by hue, saturation and lightness.

Hunting

The action of an autofocus lens which is, for one of a variety of reasons, unable to achieve a point of sharp focus. This may occur when the contrast or light level is low. Lens focus shifts continually, or hunts, between distant and close-up limits.

Hyperfocal distance

The shortest distance from a lens to an in-focus subject, for a particular aperture, when the lens is focused at infinity. When set to the hyperfocal distance depth of field extends from about half the hyperfocal distance to infinity.

Image sensor

An array of millions of tiny light-sensitive cells designed to capture and image in a digital camera.

Image stabilization lens

A lens incorporating gyroscopic sensors that detect camera shake and send compensating information to a suspended lens element to stabilize the image and reduce blurring.

Incident light

Light falling on a subject (as opposed to light reflected by it).

Infinity

A focusing point at which a lens produces a sharp image of very distant objects such as the horizon.

Infrared radiation

Electromagnet radiation, having a wavelength in the range 7,200 angstrom units to 1mm, found in the electromagnetic spectrum just beyond the red end of the visible spectrum. Although invisible to the eye it can be captured on infrared film.

Ink-jet

A printing technology based upon firing microscopic drops of ink onto a receiving medium.

Instant-return mirror

An angled mirror in an SLR camera that facilitates through-the-lens viewing but flips up to expose the film or sensor while the shutter is open.

Interpolation

The process of inserting additional pixels into an image based on the values of adjacent existing pixels.

Inverse square law

This states that the illumination of a plane surface normal to a point source is inversely proportional to the square of the distance from the source to the surface. For example, when the source to surface distance is doubled, the brightness of the surface is reduced to one quarter of its former value.

ISO

International Organization for Standardization. The ISO rating of a film or sensor represents the rate at which the light-sensitive material responds to light falling upon it.

JPEG (Joint Expert Photographic Group)

A widely-used file format designed to compress large images into a small amount of storage.

Kelvin (K)

A standard unit of temperature measured relative to absolute zero, calculated by adding 273 to degrees Celsius or centigrade, used by photographers to measure colour temperature.

Keystoning

A term used to describe the effect of converging verticals. This is caused by the image plane not being parallel to the plane of the subject, for example, when a camera is tilted upwards to record an image of a building. The effect can be avoided by ensuring that the camera remains horizontal (for a vertical subject such as a building), and corrected using the \"transform perspective\" function in Photoshop or similar editing software.

Kilobyte (KB)

One thousand bytes of computer data.

Latent image

The invisible image created by exposure of an emulsion to light, and later made visible by processing.

LCD

Liquid crystal display used by computers and equipment such as digital cameras to display images and changing alphanumeric data. It is generally a compact, low-power device.

Lens coating

Thin deposit of substance such as magnesium fluoride or silicon dioxide used to reduce the amount of light reflected from optical surfaces.

Lens speed

A measure of how much light a lens needs. A small maximum aperture equates to a slow lens, and a large maximum aperture to a fast lens.

Light-box

A box containing colour corrected fluorescent tubes which match daylight, and having a translucent plastic top, designed for convenient viewing of transparencies and negatives.

Lightness

The amount of white in a colour. Colour is characterized by hue, saturation and lightness.

Lossy

A compression technique, used to reduce the size of image files, which results in some data being removed from images. Lossless techniques compress image files without removing image or colour detail.

Low-key image

An image consisting almost exclusively of dark tones.

Lumen

A unit of light falling on a surface; used in the measurement of candle power or light output.

Macro lens

A lens whose design has been optimized for close-up work, with a maximum reproduction ratio of about 1:1.

Magnification

The ratio of the size of an image of an object to the physical size of the object. It is sometimes known as reproduction ratio.

Megabyte (MB)

One million bytes of computer data - equivalent to 1,000 Kilobytes.

Megapixel (MP)

One million pixels. The unit is commonly used as a unit of the capacity, or resolution, of a sensor in a digital camera.

Metadata

Data used to describe other data, typically intellectual content information recorded about an image to enhance accessibility.

Mid-tone

A tonal value midway between white and black. 50% grey or a value of 128 on an 8-bit scale.

Mired

Micro-reciprocal degree; a measure of colour temperature of a light quoted as the reciprocal of the colour temperature multiplied by one million. Colour correction filters commonly have "mired shift" values.

Mirror lens

A relatively compact lens that utilizes curved mirrors to form an image.

Monitor calibration

Calibration of the colours displayed by a computer's monitor using a colorimeter. This is an essential part of colour management.

Monobloc

A type of mains-powered flash head that is entirely self contained and consequently easy to use on location.

Monochrome

Strictly, an image consisting only of white, black and numerous greys. Tinted images based on any single hue may also be described as monochrome.

Motor-drive

Motorized camera system designed to automate film transport and rewind, and cocking of the shutter.

Multi-coating

Modern lenses may incorporate numerous individual glass elements used to enhance image quality. However, each additional element (glass-to-air and air-to-glass interface) reflects a small amount of light and so reduces light intensity and contrast. A transparent coating (or multi-coating) of a metal-oxide reduces this effect, minimises flare and improves contrast.

Multi-segment (matrix) metering

Exposure determination system commonly found in SLRs based upon separate measurement of brightness in at least five discrete areas of a scene; typically four quarters and a reserved central area.

Multiple exposure

A method used to combine several images on a single frame of film or in a single digital image.

Negative

An image in which tonal reproduction is the reverse of that of the subject.

Neutral-density filter

A filter that reduces light intensity without changing colour balance.

Newton's rings

Interference patterns seen when optical surfaces are placed on top of one another - typically when transparencies a held between two sheets of glass.

Noise

Unwanted signal disturbance caused by thermodynamic or mechanical instability particularly in scanners and digital cameras. Such noise tends to reduce the amount of useful data recorded.

Open flash

A method of using any number of flash discharges to increase accumulated flash output over a period of time. The camera's shutter must remain open for an extended period of time, and the method can therefore be used only when shutter speed is unimportant because of low ambient light levels.

Orthochromatic

A generic term for monochrome emulsions insensitive to red light. Monochrome printing papers are normally also orthochromatic.

Panchromatic

Light-sensitive material equally sensitive to light of all colours of the visible spectrum.

Panning

Smooth movement of a camera, during a period incorporating the time of exposure, to keep a moving subject relatively stationary in the frame.

Panorama

A broad, usually scenic, view, which extends beyond the normal human field of vision.

Panoramic camera

A camera with a wide format such as 6cm x 17cm, or equipped with a specialized scanning lens which rotates to capture a wide field of view.

Parallax

The apparent change of the position or size of an image resulting from a change of viewpoint.

Pentaprism

A five-sided prism used in SLRs to present laterally-correct viewfinder image.

Perspective

In general terms, the characteristics of a view or image determined by the relative positions of scene and viewer eg a tall building viewed from below appears to narrow towards the top.

Perspective control (PC) lens

Also known as a shift lens - a lens, with an image circle significantly larger than the frame, featuring elements which can be shifted sideways to bring selected parts of the image into view. It is typically used to correct converging verticals in architectural photography.

Petabyte (PB)

One thousand million million bytes of computer data - equivalent to one thousand Gigabytes.

Pin-hole

A small, precision-drilled hole in a metal-foil plate used as a standard aperture in a pin-hole camera. Such holes may be created using laser cutting techniques. Also, a very small hole in a photographic emulsion or in otherwise light-proof equipment, such as bellows.

Pixel

The smallest unit of a digital image, known as a picture element.

Plug-in

A software application integrated into a host program to provide additional menu options and extend capabilities.

Polarization

Broadly, the alignment or restriction of electromagnetic radiation vibrations to a particular plane.

PPI (pixels per inch)

A unit used to measure the resolution of an image produced by an input device such as a camera or scanner. The greater the number of pixels per inch the higher the resolution of the image.

Primary colours

The elementary colours which, when mixed, produce all the hues in a colour space. In an additive system they are red, green and blue, and in a subtractive system they are cyan, magenta and yellow.

Prime lens

A basic lens of fixed focal length. By comparison with equivalent zoom lenses they incorporate less design compromise and are likely to be faster, have fewer elements, and hence reduced internal reflection and losses.

Profile

A characterisation of a particular input or output imaging device used by colour management software to apply image transformations appropriate to the limitations of the device.

Quantization

The pooling of similar grey levels into a single value resulting from the limited availability of tonal resolution in a scanner. The result is that slightly different grey levels may be recorded as identical. The effect is most often seen in shadow areas of scanned images.

Quarter tones

Tones between mid-tones and highlight or shadow tones. Those falling between shadow and mid-tones are known as three-quarter tones, and those between highlight and mid-tones are known as quarter tones.

RAM (random access memory)

Used in a computer to store or access programs and data directly ie without using a sequential search process. It is therefore fast and used as a computer's primary memory.

Rangefinder

A device for measuring the distance between subject and viewer usually in order to focus a lens; hence rangefinder camera type.

RAW

A file format which stores image data as it is produced by the sensor. Conversion is necessary before editing can take place.

Reciprocity failure

A non-linear reaction to incident light of light-sensitive material. In the case of film, this generally occurs when exposures are longer than one second. Consequently, doubling the exposure time has less effect than opening the aperture by one stop.

Recycling time

The time taken for a flash gun to recharge after being fired. The time is at a maximum when the unit has been completely discharged.

Red-eye

A phenomenon revealed when light from a flash used close to the lens axis is reflected back by the subject's eyes. The reflected light is turned red or pink by blood vessels in the retina at the back of the eye, and is seen as a red circle.

Reflection

The deflection of a ray of light as a consequence of it hitting a reflecting surface. Incident and reflected rays remain on the same side of the surface.

Reflector

Typically a sheet of white, silver or gold material or card used to reflect light from a primary source back into poorly illuminated areas of a subject.

Refraction

The deflection of a ray of light as it crosses the boundary between one medium and another of a different refractive index. The ray moves from one side of the boundary to the other.

Remote release

A flexible device enabling the user to release a shutter without transferring the movement of the hand to the camera.

Reproduction ratio

The ratio of the size of an image of an object to the physical size of the object. It is sometimes known as magnification.

Resolution

In the case of a camera or scanner, a measure of ability to reproduce subject detail in an image. For printers, a measure of ability to address separate output lines or points.

Reversal film

A film which, when processed, produces positive images. It is commonly known as transparency film but also known as reversal film because one stage of the development process is designed to reverse what would otherwise be negative images.

RGB

Red, green and blue colour model which represents colours by their relative amounts of the three components. White is represented by maximum amounts of each component, and black by minimum amounts of each.

Ring-flash

An electronic flash unit consisting of one or more flash tubes formed into a near circle around a lens. It provides reasonably soft, shadowless frontal lighting suitable for close-ups.

ROM (read-only memory)

A digital memory whose data may be read but not edited.

Sabattier effect

Partial reversal of an image produced by brief exposure to light part-way through a normal development process.

Sampling

Regularly repeated measurements of small parts of a larger construct, such as an image, made with a view to reconstituting the whole from the information collected. This is the key process in digitization.

Saturation

The richness or vividness of a colour. A poorly saturated colour approaches grey; a fully saturated colour cannot be made more vivid. Colour is characterized by hue, saturation and lightness.

Scanner

Device for digitizing film-based images (film scanner) or paper-based images (flat bed scanner).

Scrim

An attachment designed to be placed in front of a light source to reduce the intensity of its output.

Second-generation image

An analogue image created from a first-generation or original analogue image, and consequently of slightly lower quality eg a print made from a negative or transparency. Copied digital images are generally identical and hence free from generational decay.

Sharpness

A rather subjective quality attributed to images displaying good contrast, and clear and distinct reproduction of detail.

Shift lens

See perspective control (PC) lens.

Shutter lag

The period between pressing the shutter release and the moment the shutter opens, particularly in a digital camera.

Shutter priority

An autoexposure mode that allows the user to select a particular shutter speed manually and leaves the camera to select a corresponding aperture.

Shutter speed

In general, the period of time during which the blades of a shutter expose a film or sensor to light.

Silica gel

A highly absorbent compound, usually white granules, used to reduce humidity. It is commonly placed in sealed bags containing equipment or film to keep the air dry.

Skylight filter

A filter that absorbs ultraviolet and a little blue light. Used to remove ultraviolet haze and improves the contrast of a distant scene.

Slide

A mounted transparency suitable for projection. Colour reversal film is therefore commonly known as "slide film".

SLR

A single-lens reflex camera featuring a single interchangeable lens through which, via a mirror and perhaps a pentaprism, a photographer is able to view an image. The mirror flips out of the light path to reveal the film or image sensor when the shutter is released.

Solarization

Broadly, the reversal of tones apparent in grossly over-exposed images (such as those of the sun).

Spectral sensitivity

The relative sensitivity of a medium to electromagnetic radiation as a function of the wavelength or frequency of the signal.

Spot meter

An exposure meter that measures reflected light from a very restricted field of view eg 1o. It has a viewfinder to facilitate accurate alignment, and a small telescope to channel light to the sensor.

Standard lens

A lens having a focal length approximately equal to the diagonal of the camera's film format, or a field of view comparable with that of the human eye ie about 45o.

Storage media

Devices designed to hold digital data typically from a computer or digital camera - eg CompactFlash card or DVD.

Strobe

An electronic flash unit usually capable of producing rapidly repeating flashes.

Sync speed

The fastest shutter speed with which flash output is fully synchronised with the period of exposure.

Synchronization (Sync)

The process of ensuring that separate events happen at the same time eg an electronic flashgun fires after a shutter has opened and before it has closed.

Teleconverter

A supplementary lens inserted between a camera body and a prime lens to increase the focal length of the prime lens by a factor of typically 1.4x or 2x.

Telephoto lens

A lens with a focal length significantly longer than that of a standard lens.

Terabyte (TB)

One million million bytes of computer data.

Thumbnail

A small, low-resolution representation of a digital image or graphic, suitable for rapid and convenient downloading or display.

TIFF - tagged image file format

A lossless uncompressed file format preferred by professionals seeking high-quality images for publication.

Tint

Subtle shades of white in a photographic image or print, perhaps determined by the presentation medium or the colour of the paper used.

Tone

The lightness of an area of an image. Highlights have light tones and shadows have dark tones.

Tracking focus

A sophisticated autofocus system that tracks continuously a moving subject.

Transparency

A film-based image or translucent print made visible by illuminating it from behind so that the eye sees transmitted light.

Tritone

An image produced by the use of three colours.

TTL (through the lens)

Through-the-lens; an in-camera system whereby exposure determination, focusing and viewing take place through the lens of a camera.

Tungsten

An artificial light source based upon heating a tungsten filament, as in domestic lamps. The colour temperature of the light emitted is about 2,800 K, and hence lower and warmer than that of daylight.

Twin lens reflex

A medium format camera using two similar lenses, one above the other. The top one is a reflex viewing lens and the lower one is used to form the photographic image. The two are coupled for focusing purposes.

Ultraviolet filter

A filter that absorbs ultraviolet light. Used to remove ultraviolet haze and improves the contrast of a distant scene, particularly at high altitudes where UV light is more apparent.

Ultraviolet radiation

Electromagnetic radiation, having a wavelength in the range 3,800 - 100 angstrom units, found in the electromagnetic spectrum just beyond the violet end of the visible spectrum. Although invisible to the eye most sensors and films are sensitive to it.

Upload

To transfer computer files to a less local or more significant location ie from a personal computer to a server.

Vernacular photography

A term used to describe photographs taken by amateur photographers, normally using point-and-shoot cameras over which they have little or no control, to record everyday events in their personal or family lives.

Vibration reduction lens

Nikon's term for an image stabilization lens.

Viewfinder

A device or system designed to indicate the extent of the field of view of a camera lens. The term is also used to describe cameras not equipped with a reflex or "through-the-lens" viewing system, and which are consequently equipped with such a device.

Vignetting

The progressive darkening of the corners of an image caused by physical limitations of lens construction or an attachment such as a filter or lens hood. The effect is also used to mask the corners of an image or make it oval or circular for presentation purposes.

Watermark

A coded identification mark embedded in a digital image, sometimes invisibly, to protect against copyright violation.

White balance

An adjustment made on a digital camera to correct for the colour temperature of the prevailing light, and hence remove colour casts from images such as those captured in tungsten light.

Wide-angle lens

A lens with a focal length significantly shorter than that of a standard lens.

X-sync

The co-axial synchronisation socket on a camera used to trigger an attached electronic flash unit when the shutter is fully open.

XD memory card

A very small memory card which uses flash memory to store digital images.

Zone system

A system of relating exposure readings to tonal values in image capture, development and printing. The system was first published by Ansel Adams.

Zoom lens

A lens whose focal length is variable over a certain range for a given aperture and focus.

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