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. AI-2