1 The Director of Photography – an overview

Film stock 63
dissolved out the still sensitive silver halide crystals, leaving stable
metallic silver as the image.
It was not until 31 January 1839 that Fox Talbot was able to present
a paper to the Royal Society in London showing he could make ‘photogenic
drawings’ or shadows of real objects. By the end of September
1840, Fox Talbot had discovered the theory of the latent image and
found a method of developing it and stabilizing it, or fixing it, to an
acceptable degree of permanence using Herschel’s sodium hyposulphate.
This technique he called the Calotype and this was the originator
of today’s negative/positive photographic process. At this stage,
though, both the negative and the positive had to be made on paper
soaked in the light-sensitive solution.
In the British Journal of Photography of 18 September 1871, an
English doctor, Richard L. Maddox, announced his method of spreading
a warm solution of gelatine onto glass plates, the gelatine being
mixed with cadmium bromide and silver nitrate. Thus, a further step
towards modern photography was taken, for now a light-sensitive
emulsion could be coated on an independent support material.
By around 1883, further developments in emulsion chemistry had
given the world an efficient dry photographic plate, made of glass,
which could be prepared long in advance of the need to use it, could
be kept for a considerable time after exposure and before development,
and required less than a second’s exposure in daylight.
By 1889, George Eastman was using a thin film of the plastic celluloid
as a base material for the film for his latest amateur still camera
and it was the introduction of this support medium, or base, that produced
the last factor needed for a film that could be used practically by
the inventors of motion picture equipment.
The early film bases were made of nitrocellulose. This produced a
very transparent and flexible support. Unfortunately, the material is
also highly inflammable and somewhat dimensionally unstable. By
1920, a ‘safety’ base using cellulose acetate became available that is
more stable, slow to ignite and burned at the same rate as paper. It is
the instability of the nitrate base that is the main reason for the poor
survival of early films. However, nitrate base continued to be used up
to the 1950s, when it was phased out.
From this potted history it is reasonable to say that four crucial elements
had to be present before a film stock capable of supporting a
moving image was available. They were:
1 The discovery of the latent image and how to develop it.
2 The application of a fixer to that developed image to give it
permanence.
3 The knowledge of how to incorporate the sensitive chemical in a
supporting medium, gelatine, so that it could easily be coated on
an independent base.
4 The introduction of a flexible support for the image, celluloid, so
that the film could run through an intermittent mechanism.
The basic photographic process
In order to consider the basic photographic process, let us look at a
simple black and white film whose physical construction will be much
as shown in Figure 6.1.