accomplished by providing a chemical reducer (de-
veloping agent) to the silver halide. The exposed
silver halide compounds are converted to metallic
silver, leaving the unexposed crystals unchanged.
The developer also contains an alkali, often known
as the accelerator, to provide the pH of 7 or higher
needed for the developing agent to function. Other
components of modern developer include agents
that reduce the amount of development that could
potentially take place in unexposed areas (known
as chemical fog). Preservatives may also be added
to reduce the reaction of developing agents to air,
resulting in oxidation (rust).
Once the image is formed, the next step is to
neutralize the non-exposed silver’s sensitivity to
light and make the image permanent. These two
things are accomplished by fixing the film in a fixing
bath. The fixer, usually a sodium thiosulfate, con-
verts the unexposed remaining silver halides into a
soluble compound that can be washed away. By
getting rid of any silver that could react later to
light (the developed image no longer has that cap-
ability), fixing accomplishes both goals. What is left
behind is the developed image.
Chromogenic (most often color) films are slightly
different in structure in that they contain dye
couplers as well as silver halides. The same reaction
takes place during exposure, but development
of these films yields the formation of dye clusters
as opposed to silver. The remaining silver is bleached
out, leaving only the dye layers intact on the film.
Density and Exposure
The resulting density on film is directly pro-
portional to the amount of exposure the film
received. The more light that strikes the film, the
more silver will form, and the resulting density will
increase. In a negative process, this means that
greater density will occur in areas of highlight, and
lesser density will form in shadows. The response of
film in this way can be diagrammed in the form of a
characteristic curve.
If one plots increasing negative density on the x-
axis, and increasing exposure on the y-axis, the
result is a curve that outlines the film’s response
to light. The lower portion of the curve, the toe, is
indicative of the shadow portions of the negative.
The straight-line portion is representative of the
film’s midtones. The shoulder, or upper portion
of the curve, where exposure and density are at
their highest, represents the highlights recorded
on film. This curve shows that density does not
increase in direct proportion to the amount of
exposure the film receives. In the shoulder and toe
portions, increase in exposure results in a smaller
increase in density than does the straight-line por-
tions. The straight-line portions show a propor-
tionate increase in density as exposure increases.
Therefore, the tonal separation of the midtones in
the negative will most accurately represent the ori-
ginal tones in the scene.
Characteristic curves vary with the contrast of
the film. The higher the contrast of the film, the
more quickly it will record greater density. The
result is a characteristic curve with a very steep
slope, with very little straight-line portion. Since
this type of film is generally lacking in midtones,
this portion of the curve is very limited.
Exposure has a direct affect on the appearance of
the film’s characteristic curve, and therefore, the
film’s contrast. For example, if a film is underex-
posed, the tones move toward the toe of the curve.
As a result, shadow areas are near to non-existent,
and midtones lack the density they need to provide
sufficient contrast. Highlights are recorded on the
straight-line portion, and represent the only tones
in the negative that will reproduce a sufficient
amount of detail.
Overexposed film will result in a shift toward the
shoulder portion of the curve. While shadows will
be well represented, midtones and highlight areas of
the negative will contain too much silver, resulting
in a loss of detail.Spectral Sensitivity
Films react differently to light than the human eye
does. Human vision is sensitive only to the visible
spectrum, which contains wavelengths from 400
(violet) to approximately 700 nm (red). The human
eye peak sensitivity is at approximately 500 nm,
which is green.
Panchromatic film, the most common type of
black and white films, has a sensitivity that extends
from outside the visible spectrum (300 nm) in the
ultraviolet range to about 650 nm (red). The result
is an increased sensitivity to blues and violets
appearing in a scene. For this reason, a clear blue
sky will appear lighter on the final black and white
image than it did in the scene.
Orthochromatic film is more sensitive to blues
and greens, but does not respond at all to reds. It is
most often used in graphic arts applications. Be-
cause it does not record light falling inside the red
wavelength, orthochromatic film can typically be
handled under a safelight without fear of fogging.FILM