PHOTOGRAPHIC SENSITOMETRY 199 



mum and maximum values of exposure for the straight-line portion of the curve, then 



L = log.o f^ (50) 



If now, the brightness range of the subject being photographed (A5) is greater than 

 L, then no matter how accurately the exposure of the negative is timed, some portion 

 of the brightness range of the original subject will fall on the curved portions of the H 

 and D characteristic and cannot be precisely and accurately reproduced, so far as 

 concerns tone value. Consequently, a third condition which must be fulfilled is that 

 the brightness range of the original subject must be equal to, or less than, the exposure 

 range R, corresponding to the straight-line portion of the negative. 



On the other hand, if the actual exposure range or brightness range of the original 

 subject is small compared to the linear region of the photographic material, appre- 

 ciable variation in absolute exposure can be tolerated without loss of fidelity of tone 

 rendering. 



We may now summarize the essential conditions which must be fulfilled to obtain 

 correct tone rendition of the original subject in the negative. Correct exposure is 

 required in order that the straight-line portion of the characteristic curve may be 

 utilized or in order that the density of all tones of the original image may be recorded 

 on the negative proportional to the corresponding logarithm of the brightness of the 

 original image. Furthermore, the brightness range of the original subject should not 

 exceed the exposure range corresponding to the linear region of the D-logio E charac- 

 teristic. Finally, in order that all tones may be correctly rendered, it is essential that 

 the negative be properly developed. Thus we can hope to obtain true tone rendering 

 in a negative only by following three fundamental axioms: (1) the negative must have 

 a linear exposure range equal to or greater than the brightness range of the subject 

 it is desired to photograph in correct tone rendition; (2) the exposure must be correct, 

 so that the image will be reproduced through the use of the straight portion of the 

 D-logio E curve; and (3) processing conditions must be such as to produce a negative 

 whose y is unity. In the last case, it is customary to say that correct tone rendition 

 requires the negative to be developed to a gamma of unity, and such developing 

 procedure is a sufficient condition. For making the final print, however, it is not a 

 necessary condition, and so far as printing is concerned, this last requirement is 

 unnecessarily severe. But as applied to the negative, as an end in itself, the above 

 three conditions must be fulfilled if proper tone rendering in monochrome is to be 

 attained. 



Characteristics of Positive Materials. — In nearly all photographic applications, the 

 negative is used merely as a means to an end. We are not accustomed to view pictures 

 in which the brightness ranges are transposed into density ranges. The usual pro- 

 cedure is to make another reversal of the photographed image by making a positive 

 or print. The process of making a positive (usually a print on paper) may be regarded 

 merely as making a negative of an image in which the brightness areas have been 

 reversed, i.e., that of making a negative of a negative. Considered in this light, it is 

 evident that the same type of arguments as have already been used for formulating 

 the conditions of correct tone rendition in the case of the negative, applj^ equally well 

 to the proper tone rendition in the case of positive prints. Largely due to inherent 

 differences between the characteristics of negatives and the corresponding character- 

 istics suitable for making positive reproductions, certain minor modifications may be 

 required. 



In discussing positive printing materials, it is desirable to differentiate between two 

 types, so far as concerns the method by which the image is viewed. Certain positive 

 materials, such as motion-picture films and lantern slides, are transparent and are 



