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").

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").

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").

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.

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.

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.

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

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

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

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.

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.

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.

Above ground level.

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

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

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.

A set of rules defining a logical or mathematical operation.

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.

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

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.

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.

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

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.

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

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.

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

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

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

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

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

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.

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

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.

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.

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

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.

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.