Glossary of Terms

A3 – a standard size for a sheet of paper or card in the ISO system having an aspect ratio of 1:√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:√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:√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.

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.

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.

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.

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.

Angstrom unit – unit, equal to 10^-10 metres, used to measure the wavelength of light.

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.

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

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 over the subject. Focusing in this way is generally faster and more accurate than manual focus.