n Ou 
did neue 
  
c re 
ace 
is highest over the longest range, its useful scale for air photography 
is considered to be the greatest. The plot of gradient against density 
(Fig.4) shows how A holds its gradient constant over a long range and up 
to relatively high densities. Although its characteristics are not ideal 
for air photography it is an advance on the general run of soft bromide 
papers and uses its density range very efficiently. 
  
  
TABLE 3 
Paper 
A B C 
Bar-Gamma 1.05 0.95 1.05 
Useful log E scale (Bar-Gamma 
limits) 1.66 1.65 |.97 
Useful log E scale (limits O.25X 
and 0.9X gamma) 1.27 1.40 0.84 
  
  
  
  
  
  
Waterproof Papers 
It has always been appreciated that waterproof bromide papers give 
inferior reproduction of fine detail, so much so that they could not be 
used for work where detail is of critical importance. Efforts to improve 
this situation have been successful and papers are becoming available in 
which the definition is little if at all inferior to that in the non- 
waterproof types. 
Electronically-controlled "Dodging" 
As recorded in the literature, North American organisations have 
developed equipment for performing automatically the shading or dodging 
necessary on most air photographs and previously done by hend adjustment 
of shading devices in the printer. Similar equipment has been developed 
in Great Britain and will soon be available commercially. Such eouipment 
is naturally more expensive than the ordinary hand printer, but not 
excessively so, and the saving ot time over conventional methods is so 
great that the initial outlay appears to be well justified. 
So far no comprehensive theoretical study of the principles of 
working of such devices and their implications has appeared, but there 
appears to be great scope for electronics in this field. 
Fig.5 shows the schematic layout of the equipment and Fig.6 shows 
for an experimental model the relationship between actual negative density 
and effective density at various settings of the feedback control. At 
maximum feedback the control is effective up to a density of 2.3; an 
original density range of 2.3 is reduced to 0.6. By progressive reduction 
of feedback, the point where control ceases moves to lower densities with 
correspondingly shorter exposure times. Very good results are being 
obtained with 10 seconds exposure time on standard bromide papers, using 
a biue tube (90-EB4-71) with typical survey negatives. 
  
   
   
    
   
   
   
   
    
  
   
    
     
      
  
  
  
    
   
  
  
  
     
  
  
    
   
MC T, UC, 
VS eU mp um e ro AR 
Cr = (C) th 
Ot rc Orr 3