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Photography (Basic) - Introduction to photography and other graphic techniques | Next
Preservative |
CHAPTER 10
IMAGE PROCESSING AND CONTROL
As a Navy photographer, you must realize that
composing and exposing a scene on film does not
guarantee top-quality photography. The quality of the
finished print depends on the quality of your darkroom
work. A perfectly exposed film is useless if it is fogged,
scratched, or under- or overdeveloped. Therefore, each
step of film processing is important and you must master
each step.
During the discussion of basic film processing
concepts, both black-and-white and color film
processing are covered. The mechanics of black and
white and color processing are very similar. The primary
differences between processing color film and
processing black-and-white film are there are more
steps in a color film process, and the time and
temperature requirements are more critical.
DEVELOPERS AND DEVELOPMENT
The purpose of development is to convert those
parts of the light-sensitive material (film or paper) that
has been affected by light to black metallic silver. This
produces a visible image from the invisible latent image.
Development is usually carried out by bringing the
exposed film into contact with a solution that contains a
developing agent, but no silver salt. The silver that forms
the developed image comes from a reduction of the
individual silver halide grains in the film emulsion. This
process is called chemical or direct development.
In another process that is seldom used, the
developed image is derived from a soluble silver salt
contained in the developing solution itself. This process
is called physical development. The physical
development process can be difficult to use because
there is a tendency for silver to be deposited where it is
not wanted.
The process of chemical development is most
commonly used for film development. Chemical
development is the process with which you should be
concerned. In chemical development, the individual
silver halide grains in the film emulsion are reduced to
a black metallic silver. Each grain in the emulsion acts
as a unit, in the sense that a grain is either developable
as a whole or is not developable. When film
development is performed properly, only exposed grains
containing a latent image are reduced to black metallic
silver. You may ask, “Why doesn’t the developer
develop the unexposed grains as well as the exposed
grains?” Actually, the unexposed grains are develop
able. When development is carried out over a long
enough period of time, all grains are developed or
reduced to black metallic silver. The density that results
from the development of unexposed silver halides is
called fog. Thus development is a rate phenomenon and
the development of the exposed grains takes place at a
faster rate than the unexposed grains.
The individual grains of silver halide in an emulsion
are protected against the action of the developer by a
chemical layer. When light strikes the emulsion, it
breaks down the protective layer at one or more points
on each individual light-struck grain. When the exposed
film is placed into the developer, the grains are acted
upon at these points by the developing agent, and each
grain that received more than minimum exposure is
quickly reduced to black metallic silver. The amount of
blackening (density) over the film surface depends
primarily upon the number of grains that have been
affected by the developer. Density is also influenced
because some grains may not develop to completion in
the time the developer is allowed to act on the film.
COMPOSITION OF A DEVELOPER
There are many different formulas used as
developing solutions, but most developers contain the
following four essential ingredients: developing agent,
preservative, accelerator or activator, and restrainer.
DEVELOPING AGENT
The developing agent, commonly referred to as the
reducing agent, is the most important chemical in a
developing solution. It is the developing agent that
actually converts the silver halide grains in the emulsion
to metallic silver. Nevertheless, the other ingredients are
necessary to make the solution function properly.
One of the properties of a developing agent is its
reducing potential. This refers to its relative ability to
develop or reduce the silver halides. An active
developing agent attacks silver halides vigorously,
whereas one of low potential is slower in its action. For
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