Films, Continued
Film speed
Film speed indicates the ability of the film emulsion to record latent images.
The International Standards Organization (ISO), a federation of all the
national standard bodies of the world, uses a uniform set of film speed
standards to rate film. This is referred to as an ISO number. This ISO
number generally looks like the following: ISO/21°. The ISO number
assigned to the film is displayed on the film canister, packing material, and
exterior packaging. Lower ISO numbers indicate slow films, while high ISO
numbers indicate faster films.
FAST FILMS: Films considered “fast” is film that requires little light for
correct exposure. These low-light film speeds have ISOs of 400 and higher.
Image resolution is good in small prints; however, pronounced graininess
appears in enlargements. Some of the very fast films, ISO/1000° and faster,
need very little light and extremely short exposure times. Fast films are
difficult to handle in bright light sitiuations but highly desirable for dimly lit
subjects.
SLOW FILMS: Slow films require more light or longer exposures to record
images. Slow films, such as ISO/25°, display very fine grain that remains
fine even in enlargements. These films are ideal in controlled studio
environments.
Spectral
sensitivity
The response of an emulsion to specific colors of light or radiant energy is
termed color or spectral sensitivity including ultraviolet and infrared radiation
as well as visible light. Color sensitivity in black-and-white films controls
the way colored objects record as tones of grey in the negative or print.
Color sensitivity determines how a film is classified into the four general
categories of monochromatic (colorblind), orthochromatic, panchromatic, and
infrared.
MONOCHROMATIC (COLORBLIND) EMULSIONS: Monochromatic
emulsions are sensitive only to UV radiation, violet, and blue light. Used for
copying and graphic arts continuous-tone and line (halftone) photography,
monochromatic films may be assigned three or more ISO values; for
example, ISO/50° for daylight, ISO/8° for tungsten light, ISO/20° for white-
flame arcs, and ISO/l2° for pulsed xenon lighting. The ISO value you use
depends upon the lighting system of your copy or process camera.
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