PHOTOGRAPHIC MATERIALS 133 



extremes of this range are used only for special purposes, while the great majority of 

 materials in general use is grouped within a range of about 3 to 1,. with the upper side 

 of this range roughly one-fourth the speed of the extreme ultrafast emulsions and the 

 lower side about six times the speed of the slow-process emulsions. 



Except for a few extremely fine-grain special copying materials, the slowest nega- 

 tive materials are the so-called "process" emulsions used chiefly in the graphic arts. 

 They have high resolving power, low graininess, high rate of development, and high 

 contrast. Thus they are particularly suited for copying black-and-white and line 

 originals and for half-tone work. The regular process materials have ordinary non- 

 color sensitive emulsions, but panchromatized emulsions are also available with the 

 general characteristics of the process materials, though with somewhat higher speed. 



The "commercial" emulsions have from about four to eight times the speed of the 

 ordinary process emulsions. They have fairly fine grain and high resolving power, and 

 the contrast can usually be varied over a considerable range by proper choice of 

 developer and developing time. They are useful for copying and for general com- 

 mercial work, particularly in the studio in cases where very short exposures are not of 

 particular importance. They are available with ordinary, orthochromatic, or pan- 

 chromatic emulsions, the speed increasing in that order. 



Above the so-called commercial emulsions there is a large group which makes up 

 the majority of negative materials, including practically all the amateur roll films, 

 as well as the films used for portrait work and for a large part of the general commer- 

 cial work. The emulsions in this group are practically all either orthochromatic or 

 panchromatic, and the speeds range from about equal to about three times that of the 

 faster commercial emulsions. The contrast and development characteristics vary 

 over a wide range, but in general the contrast is considerably lower than for the com- 

 mercial emulsions. In general, the materials in this group offer the most practical 

 compromise between the various desirable properties, such as ease of handling, reason- 

 ably high speed, considerable exposure latitude, graininess low enough for most uses, 

 good keeping qualities, etc. 



In addition to this large group of general-purpose materials there are a few ultra- 

 speed emulsions with speeds ranging up to practically four times the upper limit of the 

 general group. They are intended for use only in cases where extreme speed is essen- 

 tial, as in press photography where exposures are often made under very difficult 

 conditions. They are available with both orthochromatic and panchromatic 

 sensitizings. 



Factors Influencing Choice of Negative Emulsions. — For the great majority of 

 photographic work practically any of the fairly 'fast emulsions, those falling in the 

 "portrait" group in the classification above, will serve quite satisfactorily. For some 

 special types of work, however, certain other emulsions will be definitely superior. In 

 selecting an emulsion for any particular purpose the various properties such as speed, 

 color sensitivity, contrast, exposure latitude, graininess, and resolving power should be 

 considered, but the choice should be determined by those characteristics which are of 

 real importance in the work at hand. 



Thus, while emulsion speed is undoubtedly very important, it will often be more 

 desirable to consider how slow a material can be used, rather than to seek the fastest 

 material available, since this allows much greater leeway in selection of other desirable 

 characteristics.^ 



Somewhat connected with speed is the question of color sensitivity. Thus, for 

 exposures with tungsten light, panchromatic emulsions are necessary, except in cases 

 where long exposures can be given. Panchromatic materials are becoming increas- 

 ingly popular for general work and are necessary where various colors must be rendered 



1 Cassidt, E. H., Are Fast Films Worth the Price?, Brit. J. Phot., 84, 84 (1937). 



