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	Eng ID	Term	Definition
	1881	focal distance	See FOCAL LENGTH.
	1882	focal length	The distance from the rear NODAL POINT of a LENS to the FOCAL PLANE for an object at infinity. The distance from the surface of a MIRROR to ... more The distance from the rear NODAL POINT of a LENS to the FOCAL PLANE for an object at infinity. The distance from the surface of a MIRROR to its FOCUS. Also called focal distance.
	1883	focal length: calibrated	An adjusted value of the EQUIVALENT FOCAL LENGTH computed to distribute the effect of lens DISTORTION over the entire field used in an AERIA... more An adjusted value of the EQUIVALENT FOCAL LENGTH computed to distribute the effect of lens DISTORTION over the entire field used in an AERIAL CAMERA.
	1884	focal length: equivalent	The distance measured along the AXIS OF LENS from the rear NODAL POINT to the plane of best average DEFINITION over the entire field used in... more The distance measured along the AXIS OF LENS from the rear NODAL POINT to the plane of best average DEFINITION over the entire field used in an AERIAL CAMERA.
	1885	focal plane	A plane parallel to the plane of a LENS or MIRROR and passing through the FOCUS. In PHOTOGRAPHY, the plane (perpendicular to the axis of th... more A plane parallel to the plane of a LENS or MIRROR and passing through the FOCUS. In PHOTOGRAPHY, the plane (perpendicular to the axis of the LENS) in which IMAGES of points in the object field of the LENS are focused.
	1886	focal-plane plate	A glass plate set in the CAMERA so that the surface away from the LENS coincides with the FOCAL PLANE. Its purpose is to position the EMULSI... more A glass plate set in the CAMERA so that the surface away from the LENS coincides with the FOCAL PLANE. Its purpose is to position the EMULSION of the film in the FOCAL PLANE when the film is physically pressed into contact with the glass plate. Also called contact glass or contact plate.
	1887	focal-plane shutter	See SHUTTER.
	1888	focal point	See FOCUS.
	1889	focus	(pl. foci). That point at which parallel RAYS of light meet after being refracted by a LENS or reflected by a MIRROR. Also called focal poin... more (pl. foci). That point at which parallel RAYS of light meet after being refracted by a LENS or reflected by a MIRROR. Also called focal point. In SEISMOLOGY, the source of a given set of elastic WAVES. The true centre of an EARTHQUAKE, within which the strain energy is first converted to elastic wave energy.
	1890	focus	(v.t.). The process of adjusting an optical instrument, PROJECTOR, CATHODE-RAY TUBE, etc., to produce a clear and well-defined IMAGE.
	1891	fog	Suspension of very small water droplets in the air, generally reducing the horizontal VISIBILITY at the EARTH's surface to less than 1 kilom... more Suspension of very small water droplets in the air, generally reducing the horizontal VISIBILITY at the EARTH's surface to less than 1 kilometre.
	1892	fog: advection	FOG which forms in the lower part of a moist AIR MASS which moves over a colder surface.
	1893	fog: ice	Suspension of numerous minute ice crystals in the air, reducing the VISIBILITY at the EARTH's surface.
	1894	fog: sea	ADVECTION FOG which forms over the SEA.
	1895	fog: wet	FOG formed of droplets sufficiently large to deposit water on objects.
	1896	fog bank	FOG, generally caused by local conditions, which extends over a small area some hundreds of METRES wide.
	1897	fog bell	A bell used as a FOG SIGNAL.
	1898	fog detector light	A light used to automatically determine conditions of visibility which warrant the turning on or off of a sound signal.
	1899	fog gong	A gong used as a FOG SIGNAL.
	1900	fog gun	A gun used as a FOG SIGNAL.
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