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HANDBOOK OF PHOTOGRAPHY 



point C and logio E at point A is 2. If now we have an object whose brightness 

 range does not exceed 100 to 1, as indicated by the strip BRi, then it will be 

 possible to choose an exposure such that the densities of the negative will be directly 

 proportional to the brightness intensities of elementary areas of the original subject. 

 On the other hand, if we try to photograph on this same film a subject having a 

 brightness range of 10,000 to 1 as indicated by the strip BR«, it will only be possible 

 to reproduce correctly the range from 100 to 10,000 units representing a range of 100 to 

 1. Greater light intensities than those represented by the point C will not produce 

 densities in the negative proportional to the brightness of the object. Similarly for 

 low values of brightness of the original subject, to the left of A, a correct relation 

 between brightness of the original object and density of the negative will no longer 

 be possible. The result is that between the points A and C we are able to obtain 

 correct tone reproduction; beyond these points tone distortion results. 



Fig. 2. — If the brightness range of the subject being photographed does not exceed 

 the exposure range for which the Z)-logio E characteristic is linear, several exposures are 

 possible, all of which can give correct tone rendition. 



The ratio of the latitude of the film to the brightness range of the object determmes 

 the necessary precision of the exposure. If, as shown in BRi of Fig. 1, the brightness 

 range and the latitude are equal, then there is only one possible exposure for which 

 correct tone rendition is possible, and this is represented as the mean exposure, given 

 by the point B. If, on the other hand, the brightness range of the original subject 

 is much less than the latitude of the film, several different exposures will be possible, 

 each of which is capable of producing correct tone rendition. For example, if we are 

 photographing a subject having a brightness range of 10 to 1 on a film having a 

 latitude of 100 to 1, we may so determine the exposure that the brightness range occurs 

 between A and B, between B and C, of Fig. 2, or anywhere between these extreme 

 limits, e.g., D and E. We may, therefore, select as the mean exposure any value 

 between G and F and still obtain correct results. Since the exposure at G is 10 times 

 that at F, it is possible to make a 10 to 1 error in exposure and still obtain a negative 

 having correct tone rendition. In Fig. 1 such a range in exposure was not possible 

 because the film latitude and brightness range of the object being photographed 

 were of equal magnitude. 



