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HANDBOOK OF PHOTOGRAPHY 



Ei fall on the straight-line portion of the curve, 7 can be calculated from the relation 



(I>2 - D,) 



(log! 



E. - 



logio El) 



logio Ei/Ei 



(24) 

 (25) 



.\lthough 7 is given as the tangent of the angle a, it should be realized that the true 

 value of 7 cannot be determined by measuring (by means of a protractor, for example) 

 the slope of the curve. The reason for this is that the choice of the scales in the 

 graphical plot affects the slope of the curve as plotted but does not affect the relation 

 between the corresponding density and exposure values. From this last relation we 

 are able to calculate, for the straight-line portion of the curve, differences in density 

 produced by exposure increments, when 7 is known. Thus 



AD = 7(A logio £) = Z)2 - Di = 7(logiu ~\ 



(26) 



Inertia. — The extension of the straight line DC to the axis of density 

 defines the inertia of the photographic material. The inertia i thus represents the 



Fig. 18.- 



-Family of i)-logio E curves for different times of development. The 7 cor- 

 responding to the development time T is specified for each curve. 



minimum exposure which produces a deposit of silver, under the assumption that the 

 H and D characteristic is a straight line down to zero density. 



So far we have been considering a single D-logio E curve which, for purposes of 

 simplicity, has been somewhat idealized. However, it has already been mentioned 

 that the particular H and D curve for a given material depends, not only on the 

 photographic material itself, but also on the processing conditions. Furthermore, 

 certain of the idealized conditions which have been implicitly assumed do not always 

 occur in practice, and the deviation from the ideal case requires further study. 



To study the effects of nonidealized conditions and of processing on the character- 

 istics of photographic sensitive materials, consider the family of H and D curves of 

 Fig. 18. These curves represent a series of Z)-logio E curves as might be obtained for 



