Another  similarity  between  light  and  rain  on  a sidewalk  is  in  the  blackening  effect.  With  light  the blackening  (during  development)  increases  with  the exposure received by the sensitive film emulsion. The photographic   lens   and   shutter   assembly   should   be regarded as a device that controls the camera exposure received by the light-sensitive film emulsion inside the camera. The  aperture  of  the  lens  diaphragm  controls  the intensity of the light, and the shutter controls the time of exposure.  Since  a  photographic  reproduction  of  the original  scene  contains  a  range  of  tones  of  different brightness,   a   corresponding   range   of   photographic exposure is given to the sensitized emulsion. FILM LATITUDE A negative is said to be correctly exposed when it gives a satisfactory rendition of detail in both the deepest shadows and brightest highlights of the scene or subject. Fortunately, in many cases, there is more than a single exposure that will produce this result — there is a wide range of possible exposures within which satisfactory tone  separation  is  possible.  The  “minimum”  satisfactory exposure is one in which good tone separation is just attained in the deepest shadow areas. The “maximum” satisfactory  exposure  is  one  in  which  detail  is  just retained   in   the   brightest   highlight.   Any   additional exposure  will  cause  this  highlight  detail  to  become flattened out or “blocked up.” The range between these two exposures is known as   latitude.   This   latitude   may   be   narrow   or   wide, depending on the subject matter, lighting contrast, type of film and degree of development of the negative. In general,  the  black-and-white  films  you  will  be  using have a greater margin for error than color films. Ignoring the  influence  of  development  for  a  moment,  a  softly lighted  scene  composed  of  objects  that  are,  themselves, fairly uniform intone will allow a wide range of possible exposures that will produce a satisfactory rendition on the negative. This is caused by the narrow range of tonal values from highlight to shadows in the subject. On the other  hand,  a  brilliantly  lighted  scene  composed  of  a variety of tones, from jet black to snow white, may take up  the  entire  usable  range  of  the  negative  scale. Therefore,   the   exposure   required   for   the   proper rendition of the entire range of tonal values in this scene may be quite critical. In addition there are many scenes, such as interiors with  sunlight  coming  through  a  window,  that  have  a range of brightness so wide that no single exposure can produce  both  highlight  and  shadow  detail  in  a black-and-white negative. When you increase exposure and  reduce  the  amount  of  development,  almost  any ordinary   extreme   of   brightness   range   can   be accommodated on black-and-white film. FILM SPEED The sensitivity of black-and-white and color film for still-camera use is also called the film speed, the ISO speed  or  simply  the  ISO.  Earlier  in  this  chapter,  we pointed out that “ISO” is an acronym for International Standards  Organization,  a  federation  of  all  national standard  bodies  of  the  world,  which  has  approved  a uniform  set  of  film-speed  standards.  These  standards call for a universal expression of both arithmetic and logarithmic values with the ISO designation. Until early 1983 the emulsion speeds of still-camera film   were   expressed   in   ASA   values   (which   are arithmetic) or in DIN values (which are logarithmic). ASA  values  were  determined  according  to  standards published by the American National Standards Institute, formerly American Standards Association from which the designation ASA came. The DIN values reflected the German standards established by the Deutsche Industrie Norm. Film   speed   is   determined   by   the   manufacturer according to the ISO standards. It will generally look like this: ISO  100/21° The  number  immediately  following  “ISO”  is  the ASA  equivalent.  It  indicates  that  the  speeds  progress arithmetically,  and  any  film  marked  ISO  100  has  the same sensitivity as any other film marked ISO 100 — it is twice as fast as film marked ISO 50 and is half as fast as film marked ISO 200. The number with the degree symbol (⁰) is the DIN equivalent. The  arithmetic  speed  number  is  intended  for exposure  meters  or  cameras  marked  for  ISO  or  ASA speeds or exposure indexes. The logarithmic speed is intended  for  exposure  meters  or  cameras  marked  for ISO or DIN settings. FILM TYPES Photographic  films  (and  papers)  are  composed  of two basic parts: the emulsion and the base, or support. The emulsion is the light-sensitive portion of a film or paper that records the image. The emulsion contains the silver  halides  and  any  special  sensitizing  dyes suspended in a binder of gelatin. The gelatin holds the silver halides evenly dispersed and prevents action by a developer  until  the  silver  halides  have  been  made developable  either  by  exposure  to  light  or  chemical 11-21