EXPOSURE AND EXPOSURE DEVICES 



209 



sequent development. Consequently, the density of the silver deposit of the devel- 

 oped negative increases, at least within limits, as the amount of light falling upon 

 the emulsion is increased. The relation between the quantity of light falling upon 

 the sensitized material and the density of the silver deposit depends upon a number 

 of factors but is expressed by means of the D-logio E characteristic curve. The most 

 important of these factors are discussed in the chapter on Photographic Sensitometry, 

 which should be consulted for additional information on this subject. 



Requirements for Correct Tone Reproduction. — A scientifically correct print is one 

 in which the light intensities of the various portions of the print are proportional to' 

 the light intensities of the corresponding elementary areas of the original object. 

 Since the negative is a reversal of the light intensities of the original subject and also 

 that of the print, it follows that the opacity of the elementary areas of a scientifically 



3 r 



c 

 Q 



10.000 lOQOOO E 

 4 5 Log,oE 



Fig. 1. 



-D-logio E diagram illustrating correct tone rendition when the brightness range of 

 the subject does not exceed that of the linear portion of the curve. 



correct negative must be proportional to the light intensities of the corresponding 

 areas of the original object. In order that this condition may be fulfilled, the range 

 of light intensities of the original object must be so related to the characteristics of the 

 film, through proper exposure, that it corresponds to the linear portion of the D-logio E 

 characteristic curve. A similar condition must be fulfilled in making the print. ^ 



In addition to determining the exposure so that the linear portion of the D-logio E 

 characteristic curve is employed, it is essential that the photosensitive materials 

 selected for use have sufficient latitude to accommodate the brightness range of the 

 original object. Conversely, if the latitude of a given emulsion is small, then, for 

 a scientifically correct negative, it is essential that this material not be used to photo- 

 graph objects which have a large brightness range. 



An example will help to make these statements more clear. Let the curve of 

 Fig. 1 represent the characteristics of a film having a latitude of 100, i.e., the ratio of 

 the exposure at point C is 100 times that of point A, or the difference between logio E at 



1 In the ideal case, which is not attainable where the print is viewed by reflected light, the intensities 

 of the print would be exactly equal to the corresponding intensities of the corresponding areas of the 

 object photographed. 



^ See page 205 for treatment of correct tone rendition from the viewpoint of the characteristics of 

 photosensitive materials. 



