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HANDBOOK OF PHOTOGRAPHY 



tometric concepts by saying that the high lights of the original image (the most dense 

 portions of the negative) lack detail. But if the exposure is correctly chosen and if the 

 brightness range of the original subject is such that the entire brightness range can be 

 photographed on the linear or portion of the Z)-logio E characteristic, then all tones of 

 the original image will be proportional to the density of the corresponding photo- 

 graphed image, and correct tone rendition occurs, so far as the negative is concerned. 

 We have now to investigate another characteristic of negatives. So far we have 

 been concerned only with the fact that true tone rendition requires that the brightness 

 of the elementary areas in the original image be reproduced on the negative by opacity 

 values which are proportional to the brightness of the corresponding area photo- 

 graphed or by density values which are proportional to the logarithm of the brightness. 

 We have not inquired into the absolute value of the proportionality constant A in the 

 previous equations, which is a measure of the contrast of the negative. 



Suppose, first of all, that the opacity-brightness relations (or the density-log lo E 

 relations) are such that the brightness of an elementary area of a photographed image 



is exactly equal to the brightness of the correspond- 

 ing area of the original image, i.e., that A = 1, in 

 Eqs. (46) to (48). Then the contrast or density 

 range of the negative will be exactly equal to the 

 contrast of the original subject. If, however, the 

 density range of the negative is less than this ideal 

 value, the original brightness in the subject is com- 

 pressed, and the tones in the negative are too 

 "flat." On the other hand, if the density range in 

 the negative is greater than the ideal case, the 

 brightness of the original subject is exaggerated in 

 the negative, and the negative is said to be "too 

 contrasty." 



The contrast in negatives is its density range, 

 i.e., the range from the minimum or fog density to 

 its maximum density. If we wish to limit ourselves 

 to a consideration of the portion of the Z)-logio E 

 curve for which correct tone rendition occurs, then the density range is that corre- 

 sponding to the linear portion of the D-logio E curve. For this region of the curve, 

 we have from Fig. 17 the relation for contrast C: 



Log 10 E 

 Fig. 28. — Characteristic curve 

 of photographic materials. In con- 

 sidering true tone rendition by 

 photographic methods, it is con- 

 venient to consider only the hnear 

 region for which the density differ- 

 ence K and latitude L apply. 



C = AZ)= Z)2 - Z)i = 7(logio E2 - logio El) 



7 log:o (I) 



(49) 



This last relation shows that the contrast AD is proportional to the 7 and to the expo- 

 sure range. So long as we stay on the linear region of the Z)-logio E curve, we may 

 use 7 as a measure of the contrast obtainable; the contrast actually obtained depends 

 upon E2 and E\ as well as upon 7, however. 



It has already been shown (see page 181) that for any given photographic negative 

 material, the value of 7 depends upon the processing conditions. Specifically, it 

 has been shown that, for a given film and developer, 7 depends upon the time and 

 temperature of development. Therefore, for a given exposure range, we may control 

 contrast by the manner in which the negative material is developed (or by the value of 

 7 to which the material is processed). 



The exposure and development conditions and their efJects on the characteristics 

 of negatives may be shown graphically as in Fig. 28. This curve enables us to bring 

 out a third point not already mentioned. In Fig. 28, the exposure range for the 

 straight-line portion of the curve is L. If logio £"111111 and logio Em&s. specify the mini- 



