Depth of field:

The depth of field refers to the area within the field of view which is in focus. A large depth of field means that a large percentage of the field of view is in focus, from objects close to the lens often to infinity. A shallow depth of field has only a small section of the field of view in focus. The depth of field is influenced by several factors. A wide angle lens generally has a larger depth of field than a telephoto lens, and a higher F stop setting typically has a larger depth of field than a lower setting. With auto iris lenses, the automatic adjustment of the aperture also means constant variation of depth of field. The small depth of field is most apparent at night when the lens is fully open and the depth of field is at its minimum. Objects that were in focus during the day may become out of focus at night.

Decibels:
is a logarithmic ratio used to compare characteristics of electrical signals.

De-multiplexing:
is the process of separating the different video, audio or data channels which were multiplexed at the source.

Depth of field:
is the distance between the nearest and furthermost points of the scene which appear in sharp focus. It depends upon the F-stop and focal length of the lens.

Dielectric:
is a material which insulates the solid centre core of a coaxial cable from the shield. The thickness of the dielectric affects the impedance of the cable.

Dome:
is a type of camera housing made of smoked or tinted plastic. It is excellent for discreet surveillance and is also available with in built pan tilt heads.

DSP chip:
Digital Signal Processing chip is a solid state device which converts an analogue video signal into a digital video signal. It is used in digital cameras and also in computers to digitise video images.

Duplex:
is a system which can handle simultaneously two channels of video, audio or data signals.

Dwell time:
is a feature in a switcher. It is the time period a picture from a given camera remains on the monitor before being replaced by a picture from the next camera. Switchers may have fixed or programmable dwell time.