1 The Director of Photography – an overview

Basic sensitometry 79
Figure 7.3 Densities produced
on the negative as they relate
to original scene brightness
Density on Negative
0
Toe
Straight-Line Section
Original Scene Brightness
Shoulder
Tangent of angle � Gamma (�)
will have a greater interest in what is going on in the shadows, a knowledge
of the type and shape of this toe of the curve for the film we are
using can give the cinematographer a greater command of their storytelling
powers. At the other, higher, end of the curve, where the highlights
are going to be recorded, again the straight line rounds off to the
horizontal; this section of the characteristic curve we call the shoulder.
There is another important aspect of the characteristic curve in
which we, as cinematographers, need to take an interest. It is commonly
referred to as the gamma of the emulsion and is represented by
the Greek letter of that name – �. The gamma is the measurement of
the rate of change of the densities created as the exposure changes. It
is, in fact, a very simple measurement. One simply measures the angle
of the straight-line section of the characteristic curve to the horizontal
and takes the tangent of this angle from a scientific calculator or a set
of logarithmic tables – the resultant figure is the gamma.
The tangent of 45° is 1; therefore, a characteristic curve with a
straight-line section of 45° will have a � of 1. In this case, there will be
a straight 1:1 relationship between exposure and resultant density.
The speed of the film, or how sensitive to light it is, is often referred
to as its ASA speed. This speed rating comes from a formula devised
by the American Standards Association, hence ASA speed, but is now
more commonly referred to as a film’s ISO (International Standards
Organization).The number usually remains the same. In sensitometric
terms, films of differing sensitivity, or speed, can be expected to have
their characteristic curve in a different position on the graph paper.
Figure 7.4 shows three films that only have one differing characteristic –
their speed or sensitivity.
In Figure 7.4, the dotted lines show the different exposures required
to produce the same density on different negatives. The pale line represents
a slow film having a low ASA speed. To produce the required
density, this film needs a lot of light or a bright part of the scene. The
medium-speed film, shown as the middle of the three curves, requires
less exposure to produce the required density and the high-speed film
on the left of the three requires very little. The high-speed film therefore
will produce a medium density from a reasonably dark object,
whereas the slow film would need to be looking at a much brighter
object to produce the same resultant density.
Another characteristic the cinematographer needs to be aware of is
the relative contrast of the film in use. Contrast and gamma are often