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