1 The Director of Photography – an overview

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Basic sensitometry
For some reason, most film technicians find sensitometry either boring
or frightening. This is unfortunate, since with only a basic knowledge
of the photographic process and the ability to do your two times table,
you can master all you need to know to gain considerably more control
over your picture making.
Sensitometry, as the word would suggest, is the technique we use for
measuring and evaluating the sensitivity of a film emulsion to light.
The important part of the evaluation, to cinematographers, is to know
how much light arriving at the emulsion is needed to produce the
required density of image on the film after development.The relationship
between the amount of light, the exposure and the darkening, the
density, will not be in the same ratio over the entire range of brightness
recorded on the film.This is important, since in creating the mood
of the final picture the cinematographer will be very concerned to
know how much detail will be seen in the shadowed part of the image
and, particularly if outdoors, will wish to know the details that can be
recorded in the highlights of the scene so as to know if sky, sand, snow,
etc. will be represented faithfully in the picture. It is in the shadows
and the highlights that the response becomes non-linear.
In order to understand the relationship between exposure and
density, we commonly plot a graph of the relationship between the
two. Figure 7.1 shows a graph of exposure plotted against density.
Unfortunately, the sections of the graph crucial to the cinematographer,
the beginning and end of the curve, which represent the shadows
and the highlights, are nearly vertical and horizontal respectively
and are therefore virtually meaningless.
In 1890, two researchers, Hurter and Driffield, carried out detailed
experiments into the various characteristics of film emulsions. An
important outcome of these investigations was the realization that a
curve relating resultant image density to exposure could be redrawn
with the horizontal axis representing not exposure but, quite simply,
the logarithm of exposure.This produced a much more useful curve, as
shown in Figure 7.2. This curve, which is now universally used, is more
practical for camera exposure as it now alters by a factor that is geometric
not arithmetic. For instance, when we increase the exposure time
from one-hundredth of a second to one-fiftieth of a second, we speak
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