420 HANDBOOK OF PHOTOGRAPHY 



measure the average or integrated amount of light that gets through the entire nega- 

 tive. They will not help the photographer determine the grade of paper to be used 

 since they do not make it possible to measure small areas and thereby to determine 

 opacity differences between the high-light and the shadow portions. 



It may be said here, however, that a negative having high contrast should be 

 printed on a paper having less than normal contrast, and conversely, a fiat negative 

 requires a more contrasty paper to produce the best results. 



Practical Considerations in Negative and Positive Materials. — So far it has been 

 assumed that emulsion characteristics were perfectly straight and of sufficient length 

 to reproduce brightness ranges found in practice. Actual materials, however, are not 

 ideal; the relation between opacity and exposure in negatives is not completely linear; 

 at low values of exposure and at high values of exposure the curvature of the charac- 

 teristic means that distortion will result if these "toe" and "shoulder" portions are 

 used. 



Curves are employed to represent the relation between exposure and opacity of 

 photographic materials. In practice these relations are usually plotted as the loga- 

 rithm of the exposure against the logarithm of the opacity (called density). There are 

 two reasons for this procedure: (1) a longer range may be represented conveniently in a 

 small space in logarithmic units, and (2) the response of the eye to a light stimulus is 

 more nearly proportional to the logarithm of the stimulus than to the numerical value 

 of the stimulus. 



If a brightness ratio is to be reproduced which is so great that the curved "toe" 

 and "shoulder" portions of the D-logio .£/ curve must be used, then the extreme high 

 lights and the darkest shadows will not be truthfully translated into opacity ratios 

 (or density differences). This is because of the curvature of the relation between 

 exposure and opacity (or density). If the brightness range is small, it may be placed 

 anywhere on the straight part of the curve with the knowledge that the opacity ratios 

 (or density differences) will be proportional to brightness ratios if not exactly equal to 

 them. 



The slope of the straight-line part of the D-logio E curve, when plotted as logarithm 

 of exposure against density, is known as "gamma." It is actually equal to: 



A logio E logio {Ei/Ei) 



Gamma and contrast are often used as synonomous terms, although incorrectly. 

 The contrast of a negative is the difference between the greatest and the least density, 

 no matter whether these densities represent only the straight-line portion of the 

 D-logioi/ curve or not. Gamma pertains only to the straight portion of the curve. 

 Since, however, most photographers assume they are working only on the straight 

 portion, they use gamma and contrast indiscriminately. 



If the gamma (slope of the straight part of the JD-logio E curve) of the negative 

 is unity, the negative represents accurately the subject brightness values both in 

 proportion and in actual opacity values. The negative may be printed on a paper 

 with a gamma of unity. But if the negative gamma is higher than unity (say 1.2) 

 then the negative must be printed on a paper with a gamma of less than unity (in this 

 case 0.83) to make a print that is a true representation of the original. Mathematical 

 proof of this practical point will be found in the chapter on Photographic Sensitometry. 

 If the product of the slopes of the negative and paper curves is equal to 1, accurate 

 reproduction is procured if we neglect the extreme shadows and extreme high lights. 



With printing papers, the portion of the density-exposure characteristic that 

 is actually straight may be quite restricted in length or may be nonexistent. So long 

 as the curve has no pronounced curvature at either end or anywhere else, it is probable 



