130 



HANDBOOK OF PHOTOGRAPHY 



Resolving Power. — The ability of the emulsion to reproduce fine detail is limited by 

 the inhomogeneity of the image and the spreading of light around the edges of the 

 optical image. The resolving power is conditioned by two factors, the turbidity and 

 the contrast.^ 



The turbidity is dependent on the light-scattering power and the absorption of 

 the emulsion. Thus coarse-grain emulsions which show greater scattering tend to 

 have lower resolving power, while emulsions which have been dyed to increase the 

 absorption (so that the exposure is confined to the surface crystals) show an increase 

 in resolving power. In actual practice, the effective resolving power depends on a 

 number of factors besides the emulsion properties, particularly the contrast of the 

 optical image, the densitj^ of the image produced, and the spectral quality of the expos- 

 ing radiation. 





EASTMAN 40 



EASTMAN 33 



EASTMAN 

 PROCESS 



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Fig. 8. — Microphotometer tracings of regions of uniformly blackened areas of various 

 photographic films, showing relative graininess. The Roman numerals refer to the basic 

 emxilsions supplied with various types of color sensitizing for spectrographic work. 



Resolving power is measured by photographing a series of line gratings, the 

 numerical value being equal to the number of equal-width black and white lines per 

 millimeter that can be resolved. Since the resolving power depends on so many 

 external factors in exposure and development, the absolute value is of much less 

 importance than the relative values for different emulsions. The average resolving 

 powers for a group of emulsions covering a considerable range of speed and contrast 

 are shown in Table III. These values were obtained with an optical image contrast 

 of 20 and for the density showing maximum resolving power when development was 

 carried to give y — 0.8 y^. 



Negative Emulsions. — Because of the extremely varied types of work which must 

 be handled in the various fields of photography, negative emulsions with widelj^ vary- 

 ing characteristics are needed and have become available. The chief variations are 

 in spectral sensitivity, speed, contrast, graininess, and resolving power. 



Spectral Sensitivity. — As mentioned previously, ordinary emulsions are sensitive 

 onlj'^ to blue and ultraviolet light, orthochromatic emulsions are sensitive also to green, 

 and panchromatic emulsions are sensitive to all colors of the visible spectrum. 



Ibid. 



