178 



HANDBOOK OF PHOTOGRAPHY 



region of correct exposure^ (CD) in which densitj^ is proportional to logio E, the region 

 of overexposure (DE), and the region of reversal (EF), since in this last region density 

 decreases rather than increases with increasing exposure. 



In the case of many photographic materials, it has been found experimentally 

 that a considerable portion of the H and D curve is represented satisfactorily, within 

 the limits of experimental error, by a straight line. The linear relation between the 

 logarithm of the exposure and the density is of utmost importance in the correct 

 rendering of tone values in black-and-white photography and in the correct rendering 

 of both tone and color in color photography. For the correct proportional rendering 

 of the various object brightnesses in the negative, the camera exposure is usually 

 adjusted so that the straight-line portion of the Z)-logio E curve, i.e., the region 

 between C and D, is used. For the fulfillment of this condition the minimum density 

 3.5 



Fig. 



-Typical D-lo 



2.0 1.0 12 3 4 5 6 



Log ,0 

 E or H and D characteristic curve for photographic materiE 

 with important regions specified. 



in the negative (corresponding to the deepest shadow in the object) must not be less 

 than that of point B, and the maximum negative density (corresponding to the highest 

 light in the object) must not exceed that of point E. 



Latitude. — Projection of the points C and D on the two rectangular axes determines 

 the range in density as well as the exposure range for which linearity between density 

 and log E exists. The exposure range for which this linearity exists is called the 

 latitude of the photographic material. Thus latitude is defined to be 



L = 



logio E. 

 E2 



Ey 



login Ex 



E, 



log.o pT (in logio E units) (16) 



-C-l 



(in exposure units) (17) 



1 The linear portion of the Z)-logio E curve is called the region of correct exposure because it is 

 usually desired to have the opacity of the negative directly proportional to the exposure. Thus the 

 u.sually desired condition is that for which = kE. Since D = A -\- logio O, the usually desired 

 condition requires that relation between density and exposure be 



D = A + logio k + logio E = B + logio E 



which condition is fulfilled by the linear region of the H and D curve. 



Of course, it may sometimes be desired to introduce certain forms of distortion intentionally in 

 order to attain artistic or other desired effects. In this case, the straight-line portion would not neces- 

 sarily represent the region of correct exposure. 



