CHAPTER VIII 



EXPOSURE AND EXPOSURE DEVICES 



By Bevekly DtTDLEY AND A. T. Williams 



Exposiire. Theoretical Considerations. — Two fundamental steps are involved in 

 the making of any picture by photographic processes which involve the use of silver 

 salts. In the first of these a "negative" is produced on which is recorded, in reverse 

 manner, the light and dark portions of the original subject. In the second step a 

 positive print is made by using the negative as the original object, and reversing its 

 tone shades so that the brightness of the various elementary areas of the print bear 

 some direct or positive relation to the brightness of the corresponding elementary 

 areas of the original object. In both steps, the process in which light falls upon the 

 sensitized photographic material is an important step and is known as exposure of 

 the photographic material. 



In the final analysis, the positive print is the desired result. Any intermediate 

 steps, such as those involving the making of the negative, must be regarded as a 

 means to the desired end and, accordingly, should be treated in this light. Since the 

 final print is produced from the negative, it is evident that the characteristics of the 

 negative, which include its general or average density, its range of density, and its 

 tone gradation, will influence the printing time, range of density, and tone gradation 

 of the final print. For this reason it is essential to ascertain that the negative is 

 properly prepared in all respects; in this chapter, however, we shall be primarily con- 

 cerned with the proper exposure of the negative, leaving its subsequent processing to 

 later chapters. The manner in which the negative and the positive print are exposed 

 to light, the duration of the exposure time, the characteristics of the photographic 

 material, the spectral characteristics and intensity of the light source, the char- 

 acteristics of any filters which may be used, are all important factors to consider in 

 arriving at the proper exposure which will produce the desired final results. This 

 chapter will be devoted to a consideration of these factors. 



Photochemical Action Occurring during Exposure. — The emulsions of photographic 

 materials consist of a layer of silver halide deposited on a transparent substance such 

 as gelatin or glass in the case of negative materials or on a paper base in the case of 

 positive materials. It is characteristic of the silver halides that, when acted upon by 

 light, they undergo some form of alteration so that the grains which have been 

 exposed to light may be converted into black deposits of metallic silver in the process 

 of development.^ 



A minimum amount of light, however, is required to effect the change in the grains 

 of the silver halide. If less than this amount of light fails to reach the emulsion, the 

 grains will not be affected, and consequently they cannot be changed into metallic 

 silver during development. As the quantity of light impinging upon the emulsion 

 increases beyond the minimum amount necessarj^ to produce a minimum observable 

 silver deposit, more and more layers of silver halide are exposed and are subject to sub- 



' The exact physics underlying the production of the latent image in the emulsion need not concern 

 us here. There is no universal agreement as to the mechanism by which the latent image is produced, 

 and this point will not be discussed here. 



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