Glossary of Terms

DAC (digital to analogue conversion) – the process of changing a series of representative digitized values into continuously varying analogue data.

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 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.

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.

D-SLR – a single-lens reflex camera that uses a digital sensor rather than film.

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.