Let us take these procedures in order. The first
decision which has to be made is whether to photograph or
not. The decision is made on a guessed estimation of
the brightness range, whereas if the brightness range
could be measured, it could be specified that photography
should not take place unless the brightness range exceeded
an agreed minimum and by this means a numerical limit to
the photographic quality could be set so far as this is
determined by haze.
2.2. The next decision to be made concerns the type of
emulsion to be used. This also depends on a guessed
estimation of the subject brightness range. This guess
is generally made at the home base headquarters on the
basis of the brightness conditions anticipated in a
particular country, and sometimes expeditions are equipped
with a type of film suitable for only one type of condition.
2*3* The next decision to be made is concerned with
exposure, but since the exposure depends on the development
subsequently given to the film, it is necessaiy first of
all to consider current practice with regard to develop
ment. With only few exceptions development in the field
is carried out in roly-poly spool tanks such as have been
in continuous use since at least 1927« In spool tanks,
development to a specific gamma varies not only with time
and temperature, but also with the winding speed and the
length of the film. Nor is it easy to stop development
abruptly. When using spool tanks one therefore requires
a good margin of latitude on development time if results
are to be acceptable. This may be achieved only by
developing right out to where the gamma-time curve is
flat, find development time is not critical. Usual
procedure is thus to develop right out. By this means
contrasts lower than 1.3 a*e unobtainable, and higher
contrasts may only be achieved in jumps according to the
performance of the various available films. Much versa
tility is thereby lost, and optimum results are only
possible when, by luck, the maximum contrast set by the
film happens to match the subject brightness range.
Where development contrast is thus fixed, the best
one can do is to expose so that the middle brightnesses
photograph in the middle density range* This is the aim
by usual practice, and the exposure is estimated for
average scene brightness by guess, or by the use of an
integrating exposure meter. If the exposure is correctly
estimated, detail in the middle brightness range will
photograph, and that is about all one can be certain of
by usual procedures.
2.4* By the procedures just described, the resulting
negatives may vary between a condition which is too flat
for adequate separation of fine detail, to one vrtiere the
middle brightnesses exhibit extreme contrast, and the
remaining brightnesses are entirely outside the range of