1 The Director of Photography – an overview

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Figure 6.5 After
development
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Still in total darkness, and after exactly the right amount of development
time, we plunge our film into an acid stop bath. As its name
suggests, this immediately stops the process of development but still
leaves the image vulnerable to more light. After the acid stop bath,
there remain active silver halide grains in the film so, still in complete
darkness, the film must now go into a fix bath, which turns the remaining
silver halide into substance (complex sodium argentothiosulphates)
that can then be washed out of the emulsion without affecting
the metallic silver.This chemical, the fixer, is the sodium hyposulphate
discovered by Sir John Herschel in 1839, referred to earlier in this
chapter. You will occasionally hear the fixer referred to as ‘hypo’, a
name derived from the chemical name of its primary component. So,
if the fixer dissolves out of the photographic emulsion all the undeveloped
grains but leaves all the developed grains completely unaffected,
then our little microcosm of the film will look like Figure 6.6 and will,
at last, be safe to bring into the light. The application of all the above
liquid chemicals to the emulsion is referred to as processing that emulsion.
If you compare Figures 6.2 and 6.7 you will see the effect of processing
on the whole grain field.
Now the number of photons in the above example might be considered
a medium exposure. Were we to consider a higher exposure,
which would be like that found in a much brighter part of the overall
scene, then the same four grains we used in the above example would
be attacked by a photon array much like that shown in the left of
Figure 6.8, but after processing all but one of the grains will be black,
as only the smallest grain failed to have four atoms of silver halide
turned to metallic silver. In Figure 6.9 we see the reverse, where a dark
part of the scene is recorded, and as there are far fewer photons only
the largest grain is fortunate enough to have the requisite four atoms
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