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HANDBOOK OF PHOTOGRAPHY 



in a reasonably true tonal relationship or where orange or red filters are used. They 

 are particularly valuable in portrait photography since they avoid the exaggeration of 

 skin defects given by blue-sensitive materials and thus minimize the need for 

 retouching. In many cases, however, particularly for daylight exposures, ortho- 

 chromatic or even ordinary materials will be quite satisfactory and may be desirable. 

 Thus some landscape workers prefer these to panchromatic emulsions because the 

 greater reproduction of blue atmospheric haze increases the apparent separation of 

 planes and enhances the "aerial perspective." 



The user often does not give much consideration to the contrast characteristics, 

 probably because with many emulsions the contrast can be varied over a considerable 

 range by proper control of exposure and development (Fig. 9) and because the range 

 of contrasts obtainable in printing materials makes it possible to obtain acceptable 

 prints from negatives of widely varying contrast. In some applications, however, the 



4.0 



3.0 



2.0"" 



1.0 



1.0 



40 



5.Q 



6.0 



2.0 3.0 



Log Exposure 



Fig. 9. — Graphs illustrating the fundamental characteristics of negative materials. 

 The large curve shows the D-logio E characteristic, sometimes referred to as the H and D 

 curve. The smaller graph shows the manner in which fog and gamma increase with time 

 of development. 



contrast characteristics are very important. This is particularly true with the various 

 processes of color photography, where a low-contrast negative which will not exceed 

 the exposure range of the printing process .is usually needed. Also in copying line 

 originals the high contrast desirable to offset the inescapable losses from such sources 

 as lens flare can only be obtained with some of the special high-contrast materials. 



Exposure latitude is to a considerable extent dependent on contrast. Thus very 

 high contrast materials can obviously have very little exposure latitude. Fortunately 

 this is not serious as in practically all cases where very high contrast is needed the 

 exposures are under precise control. For normal work, however, particularly with 

 daylight exposures, a certain amount of latitude is very desirable. Fortunately, most 

 of the modern double-coated emulsions have even more latitude than is often used. 



Graininess and resolving power have become of considerable importance since the 

 wide adoption of the miniature camera. Obviously, with any of the small negatives 

 which require many times enlargement, the graininess is very important. With the 

 larger sizes of negatives, however, which will be used only for contact printing or for a 

 limited degree of enlargement, the graininess of most modern materials is so low that it 

 can be disregarded, at least until all other desirable characteristics have been obtained. 



