Many substances are affected in some way by light.
The light-sensitive substances used in photographic film
to record an image are silver salts and are called silver
halides. The silver halides react to ultraviolet radiation,
violet, and blue light only; however, they can be made
sensitive to other colors of light and infrared radiation
by the addition of dyes. Depending on the amount of
light and the type of silver halide, the light produces a
visible or invisible change in the halides of a film or
printing paper. An invisible change is made visible by
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 action. Also, the
gelatin acts as a sensitizer for the silver salts.
In photographic films and papers, the primary
purpose of the base is to support or hold the emulsion in
place. The base, or support, may be transparent or
opaque, depending upon how the recorded image is to
be used. A transparent base is used for transparencies
that are viewed by transmitted light and for negatives
that are printed with transmitted light. An opaque base
is used for prints that are viewed by reflected light.
The latest state of the art in light-sensitive materials
used in photography is the use of the electronic medium.
Video disks do not contain an emulsion or a base. When
electronic mediums are used, light is converted to
electrical impulses and these impulses are stored
magnetically on a tape or disk. Since it is the camera
itself that converts light to electrical impulses, the
recording medium and all stages of the photographic
process can be carried out in normal room light.
The characteristics and use of black-and-white film
depend largely on the actual construction of the
emulsion. These characteristics include the following:
the degree of sensitivity to light, response to various
colors of light (color or spectral sensitivity), contrast,
exposure latitude, emulsion latitude, and emulsion
There are many types of black-and-white films
available. Each type differs somewhat from the others.
You should become acquainted with the characteristics
of films. This knowledge is helpful in selecting the film
most suitable for each photographic assignment.
EMULSION SENSITIVITY TO LIGHT
The silver halides and sensitizing dyes of most film
emulsions are very sensitive to small amounts of light.
This light causes invisible changes to the emulsion and
is called the latent image. The latent image can be
physically made visible by the chemical step of
development. The extent of the reaction to the light of
the emulsion is affected greatly by the size of the silver
halide grains and the amount of light reaching the film.
The inherent property of a film emulsion to respond to
light is termed film speed.
Film speed is important, since it is related to the
amount of exposure required to produce an acceptable
image. Emulsions are rated as slow, medium, or fast,
depending on the amount of light required to produce an
image satisfactorily. Fast emulsions require less light to
produce an acceptable image than slow emulsions.
To calculate the exposure for a film emulsion
accurately and consistently with a light meter, the
manufacturer has developed a system of rating emulsion
speed. The rating system used is the IS0 film speed
system. ISO film speed is a numerical value assigned to
an emulsion used for determining exposures.
The International Standards Organization (ISO) is
a federation of all the national standard bodies of the
world. It has approved a uniform set of film speed
standards. The standards call for a universal expression
of both arithmetic and logarithmic speed values with the
ISO designation. The ISO designation generally looks
like the following: