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.

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.

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.

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

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.

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

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

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.

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.

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

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

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.

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

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

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.

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