2. 9 
Corrections to the table for film speed variation 
Since the speed of a film may vary from batch to 
batch it may be necessary to determine the 3peed of a 
sample from each batch before an expedition leaves its 
home base. Appropriate corrections can then be made to 
the working table. 
2.10. The Negative density working range 
Something should perhaps be said about the positions 
of the levels M and N in figure 1. The lower level N 
must clearly be above fog, and a value of 0.2 above fog 
has been suggested. There is less agreement with regard 
to the position of the upper level M. Some would place 
this as high as 1.6. Others, notably K. B. Jackson, 
would place it as low as 1.2Further research is 
necessary to establish the best position of these levels, 
and for this purpose it will not be sufficient to assess 
resolution with the usual 0.2 contrast difference chart, 
because Carmanw) has shown that in the aerial scene the 
log brightness difference between adjacent small details 
is more in the order of 0.08 and can be as low as 0.02. 
The writer would observe also, that if the end 
product is to be a print, a density range between fine 
details much over 1.0 cannot be accommodated on paper by 
any means, and only on a transparency by the use of soft- 
grade materials. The dodging printers assist only when 
large density differences occur between large areas of 
detail. Where large density differences occur abruptly 
or between small details the dodging printers are of no 
help. 
2.11. Some interesting conclusions with respect to exposure 
variation 
It may be observed that minimum brightness will 
always be found in the shadows, and maximum brightness 
on light textured surfaces in direct sunlight. Dark 
shadows, and light textured sunlit surfaces are to be 
found in any aerial subject. Thus with possible few 
exceptions such as the sudden appearance of snow, which 
is brighter than any other natural details, a change in 
either maximum or minimum brightness is likely to be 
consequent only on a change of sun altitude, or of the 
amount of obscuring matter (haze) between the camera 
and the subject. A significant change of sun altitude 
cannot occur instantaneously, nor is it likely that the 
haze condition would change abruptly in this manner; 
though an exception would be when running into indus 
trial haze. In consequence it may be said that 
excluding snow or unusual conditions the necessity for 
changing exposure abruptly when running from say open 
country into forest, arises only when the photography is 
not embracing the whole brightness range equal for both 
scenes.