1 The Director of Photography – an overview

Basic sensitometry 81
Figure 7.6 The effect of
changing the gamma of an
emulsion
Density
Log of Exposure
Film with a high Gamma
Film with a medium Gamma
Film with a low Gamma
contrast ratio. As can be seen by comparing the curves in Figure 7.6,
by increasing the gamma and steepening the curve the usable straightline
section relates to a shorter distance along the exposure axis, thus
the contrast ratio will be lower. We must learn to recognize the difference
between contrast expressed as a ratio and apparent contrast as a
‘soot and whitewash’ image.
In all the graphs shown in this chapter you will see that the curve
never reaches zero on the vertical density line. This is because the film
can never be made absolutely clear, i.e. having a density of zero.This is
mainly the result of two things:
1 The base material it is coated on is not completely clear.
2 When film is manufactured it receives an almost imperceptible
exposure from things other than light. The film emulsion is very
slightly sensitive to pressure, heat, etc. and therefore the actual
process of manufacture causes a very slight darkening.
The resultant small density at the very bottom of the curve we therefore
call the base level fog.