Film Processing
Photographic film processing converts the exposed but invisible image (the latent image) into
a visible one. This step is mandatory before a re-creation on film of the original scene is
possible. Although processing is usually carried out in total darkness, some materials used in
black-and-white photography are suitable for use with an appropriate safelight--a light source
of a color that does not affect the sensitive material.
BLACK-AND-WHITE FILM
In order to process black-and-white negative film, three solutions are required. The first and
most important, the developer, converts the exposed portion of the silver halides in the film
emulsion into black metallic silver, which is what actually forms the image. The amount of
time the film is immersed in the developer solution is critical and must be accurately
measured. The second step is the stop bath, or rinse. Its function is to stop the action of the
developer remaining in the film. This helps to control the exact period of time the developer is
allowed to work. The stop bath also helps to prevent the formation of bubbles of carbon black
spots in the final print.
The film is next placed into a fixing bath. This solution removes the unused silver halides in
the film by forming a soluble solution of a silver salt, rendering the image permanent and
insensitive to light. The fixer must then be removed either by washing the film in running
water or by using some form of chemical neutralization. If this is not done, the constituents in
the fixer will, in time, cause the image to fade. After washing, the film is immersed in a
wetting agent, usually ethylene glycol, which reduces the surface tension of the water
remaining on the film and helps it to dry without leaving any spots or marks. Finally, the film
is placed in a dust-free enclosure to dry; forced air or heat may be used to accelerate the
drying step.
The most convenient way to handle roll film, such as 35 mm, is to wind it around a spiral
developing reel made of stainless steel or plastic. The reel is then placed into a light-tight tank
that is equipped with a special lid which allows the processing solutions to be poured in and
out under normal room light.
COLOR FILM
Color processing requires more equipment than does black-and-white, because color films are
far more complex than their black-and-white counterparts. For color film, an accurate
thermometer is needed to monitor solution temperatures. Some color films have tolerances as
low as plus or minus 0.45 deg C (plus or minus 0.25 deg F). In color negative film processing
the function of the developer is almost identical to its function in standard black-and-white
processing. It converts the exposed silver halide grains in the film emulsion into metallic
silver. Wherever metallic silver is present, the developer interacts with color-forming agents
in the three different layers to form color dyes. The next step is the bleach, which stops the
development process and converts the metallic silver back into silver halide. The fixer step
removes all of the soluble silver salts in the film. The film is then washed to remove traces of
fixer. Next, a stabilizer is added, which hardens the emulsion and extends the life of the color
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dyes. The stabilizer solution also contains a wetting agent, which helps prevent drying marks
on the negative.
POSITIVE PRINTS
A positive print is generally made from a negative by placing the negative in an ENLARGER,
which is essentially a projector that focuses the negative image at a specified degree of
magnification, for a specified period of time, through a lens, onto a piece of sensitized
photographic printing paper. An alternative procedure is to make a contact print: the emulsion
side of a negative is pressed into contact with the emulsion side of a piece of printing paper;
light is then passed through the negative to expose the emulsion, resulting in a print that is the
same size as the negative. The remaining steps are the same for both methods.
The exposed paper is put into the developer solution, which converts the exposed silver
halides into metallic silver. With color film, the dye couplers combine with the developer to
form the color image wherever the metallic silver is present. The bleach-fix step converts the
metallic silver into a soluble silver salt solution and removes it, along with any remaining
unexposed silver halide grains. The paper is next washed to remove any trace of fixer and
then dried. Black-and-white prints are put through the same processing steps as black-andwhite film: develop, stop bath, fix, wash, and dry.
The actual process can be carried out in a variety of ways. A tray system involves a series of
shallow dishes containing the chemical solutions. The paper is dipped in each tray
successively for the specified period of time and then laid flat to dry. In the drum method the
paper remains inside a light-tight cylinder while the solutions or wash water are poured in and
out. After processing is completed the paper is removed for drying.
For large-scale or industrial film and paper processing, modern automatic processing
machines are used. Exposed materials are inserted at one end, and processed film or paper that
is dry and ready to use emerges at the other end. Developing, rinsing, fixing, washing, and
drying are all handled automatically within the machine.
Martin Hershenson
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