ABERRATION A defect in the formation of an optical image; for example, astigmatism, chromatic aberration, curvature of field, and so forth.

ABSOLUTE TEMPERATURE The temperature measured from absolute zero. Expressed as degrees Kelvin (K) in the Centigrade system where

absolute zero is 273C or in degrees Rankine in the Fahrenheit system where absolute zero is 459F on the scale.

ACCELERATOR Chemical constituent of photo graphic developers that activates the developing agent and swells the gelatin to hasten penetration of the solution. See SODIUM HYDROXIDE, SODIUM CARBONATE, SODIUM META BORATE, and SODIUM BORATE.

ACHROMATIC COLORS Colors perceived as having no hue (white, black, gray, and silver).

ACHROMATISM The absence of chromatic aberration.

ACID, ACETIC A colorless liquid of pungent odor used in stop baths and in fixing baths. In concentrated form it attacks the skin and produces painful blisters. A concentrated solution of 99% solidifies at 62F and forms a mass resembling ice (Glacial Acetic Acid).

ACID, BORIC (BORACIC ACID) H3BO3Colorless, odorless, transparent crystals, or a white amorphous powder. Slightly soluble in water and more soluble in glycerine and alcohol. Used in toning and fixing baths.

ACID, SULFURIC, H2SO4 syrupy, odorless liquid, colorless or slightly yellow. Used for preparing a tray cleaning solution and in fixing and reducing solutions.

ACTION Movement within a scene being photo graphed. Also, the picture portion of a motion picture as differentiated from the sound track portion.

ACUTANCE An objective measure of the ability of a photographic material to show a sharp line of demarcation between contiguous areas receiving low and high exposures. It correlates well with

subjective judgments of picture sharpness. It is the mean of the square of the density gradients times the density scale over a boundary.

ADDITIVE PROCESS Any color process in which a reproduction is formed by a combination of images each of which supplies color in proportion to the color observed in the original scene. In a typical, three color additive process, the colors of the images are blue, green, and red. See SUB TRACTIVE PROCESS.

AGC Automatic gain control. Regulates the volume of the audio or video light levels automatically within a camcorder.

AGITATION The act of moving a photographic film, plate, or paper in a processing bath or moving the bath relative to the photographic material during processing.

AIR BELLS ( 1) Air bells are bubbles of air that prevent contact between a processing bath and localized areas on the surface of a photographic material. (ASA) (2) Undeveloped spots on negatives or prints caused by air bubbles, preventing access of developer.

ALKALI A water soluble compound capable of uniting with and neutralizing acids. The alkalies commonly used for photographic processing baths are sodium hydroxide, potassium hydroxide, sodium carbonate (monohydrate and anhydrous), potassium carbonate, sodium tetraborate, sodium metaborate, and ammonium hydroxide.

AMBIENT SOUND Background sound or wild sound. Sound that surrounds the scene or location, received by the microphone and recorded onto magnetic tape.

AMMONIUM THIOSULFATE, (NH4)2S2O3 A white salt freely soluble in water. Used in the preparation of rapid fixing solutions.

ANALOG An analog signal that fluctuates exactly like the original stimulus (for example, sweep second hand clock, phonograph player).

ANGLE OF ACCEPTANCE The angle that objects must align within to affect the reading of a photoelectric exposure meter.

ANGLE OF FIELD A property of a lens. The angle subtended by the lines that pass through the center of the lens and locate the diameter of the maximum image area within the specified definition of the lens. Also called angular fields.

ANHYDROUS Dry, containing no water of crystallization.

ANTICURL BACKING A transparent, gelatin coating sometimes applied to the opposite side of a photographic film from the emulsion to prevent curling by balancing the forces that tend to curl the film, as it is wet and dried during processing.

ANTIHALATION COATING A light absorbing coating applied to the back side of the support of a film or plate, or between the emulsion and the support, to suppress halation (also called antihalation backing). See HALATION.

ANTISLUDGE AGENT A chemical compound added to photographic processing solutions to prevent the formation of sludge. Sodium metaphosphate and boric acid are commonly used for this purpose.

APERTURE In an optical system, an opening through which light can pass.

APERTURE, CURTAIN The slit in a focal plane shutter that permits light to reach the film. The slit size may be either fixed or variable.

ARTIFICIAL LIGHT Illumination provided by incandescent, fluorescent, or flame sources.

ASPECT RATIO The ratio of the height to the width of the film or television frame; that is, three units high to four units wide (3:4).

ASTIGMATISM A lens aberration that causes an off axis point to be imaged as a pair of lines at right angles to each other and in different focal planes. A lens having astigmatism is unable to image horizontal and vertical lines in the same plane with equal sharpness.

ATMOSPHERIC PERSPECTIVE Applied to the effect of distance created by atmospheric haze in a photograph. It lightens the tones as the distance increases.

AUDIO TRACK The area of the videotape that is used for recording audio information.

AUTO IRIS An automatic control of the lens diaphragm.

AUTOFOCUS A feature of certain cameras or enlargers by which the image is kept in focus automatically regardless of the degree of reduction or magnification.

AVOIRDUPOIS The system of weights commonly used in the United States and the British Empire in which the primary unit is the pound (7,000 grains); usually expressed in pounds, ounces, and binary fractions thereof.

BACK LIGHT Illumination from behind the subject in a direction substantially parallel to a vertical plane through the optical axis of the camera.

BACKGROUND (1) That part of the landscape which is more distant than the principal object from the camera. (2) A screen, drape, or projected scene used in a photography studio behind the subject.

BACKING PAPER (ROLL FILM) The protective strip of paper to which the film is attached. Backing paper is usually black on one side and colored on the opposite side. Numerals are usually printed on the colored side in a position where they can be viewed through the camera window. Also called duplex paper. (ASA)

BARN DOOR Folding wings used in front of studio spotlights to aid in directing the light and to shade portions of the subject from direct illumination.

BASE DENSITY The density of a film base. No plastic is 100% transparent, so all films have some base density.

BATH Any chemical solution used in processing photographic materials.

BEAM SPLITTER An optical system so arranged as to reflect or transmit two or more portions of a light beam along different optical paths.

BELLOWS The extensible lightproof device that joins the lens board to the film support section of a camera

BLEACH, PHOTOGRAPHIC (1) To remove an image from a photographic film. Especially to do this by converting a metallic silver image to a halide or other salt that can be removed from the film with hypo. When bleaching is not carried to completion, it is called reducing. (2) Any chemical reagent that can be used for bleaching. (3) Any chemical solution used for bleaching.