196 HANDBOOK OF PHOTOGRAPHY 



Nevertheless, the Eastman Kodak Co. has recently adopted a method of film- 

 speed rating based on minimum useful gradient. ^ This method involves drawing the 

 characteristic curve, and selecting the exposures at which the slope (or gradient) of 

 the curves is 0.3 that of the average gradient for a log exposure range of 1.5, the origin 

 of this exposure range being taken at the threshold value. 



Comparison of Speed Ratings in Common Use. — Before closing the subject of the 

 speed or sensitivity ratings of photographic materials, it may be well to compare the 

 ratings of the various methods in common use. Such a comparison between ratings 

 based on various modes of determining speed has no justification, from theoretical 

 considerations. But intercomparisons of the various film-speed ratings has some 

 practical utility, in these daj^s of elaborate exposure meters, when the film speed is 

 given in one mode of rating and the exposure-determining device is based on some 

 other system. The practical photographer is then faced with the problem of deter- 

 mining the method of transposing from one system to another. 



Appendix B gives the comparative ratings of the more important film-speed rat- 

 ings in use at the present time. The data in this table may be considered to represent 

 average conditions fairly well. The characteristics of some particular emulsion in 

 question may be such as to prohibit use of this Appendix B for comparisons with any 

 degree of precision. Consequently, for any specific situation, the translation from 

 one speed rating to another can be used only as a rough approximation or as a guide to 

 the correct order of magnitude. 



Some Practical Applications of Sensitometry. — In its scientific aspects, photo- 

 graphic sensitometry is of considerably more importance to the manufacturer of photo- 

 graphic materials, the motion-picture laboratory technician, the astronomer, the 

 physicist, the color engraver, and the advanced worker in color photography than to 

 the commercial or amateur photographer. Yet even the least scientifically inclined 

 photographer makes some use of sensitometric principles when he makes a successful 

 negative or print, and a complete understanding of the significance and technique of 

 sensitometry will aid in making better photographs. It is the purpose of this section 

 to discuss some of the more practical aspects of sensitometry and the application of 

 sensitometric concepts to the making of photographs having proper monochrome 

 rendition of tone. 



Characteristics of Negatives. — It has been shown that the D-logio E characteristic 

 is one of the most important in specifying the stimulus-response relation of photo- 

 graphic materials, especially if we limit ourselves to a discussion of monochrome 

 photography. This characteristic may be used in a qualitative sort of way, to detect 

 defects in exposure or processing of negative materials. The methods for so doing 

 may be applied by anyone who has a proper understanding of the philosophical signifi- 

 cance of photographic sensitometrj'-, even though he does not possess measuring 

 equipment. 



In exposing a negative, the relation between the brightness of the original subject 

 being photographed and the brightness of the film when viewed by transmitted light 

 has been reversed; it is for this reason that the film is called a negative. The brighter 

 the original subject, the darker (the more dense) will be the resulting negative, for the 

 same time of exposure. Similarly, the brighter portions of the subject will show up on 

 the negative as denser or more opaque portions in the representation of the original 

 subject. There is consequently some sort of correspondence between the brightness of 

 the elementary areas of the original subject and the opacities of the corresponding areas 

 as seen in the negative. In order that the negative may portray properly the original 

 subject, it is necessary that the brightness of the elementary areas of the original subject 

 be directly proportional to the opacity of the corresponding elementary areas as 



1 Clark, Walter, The Problem of Film Speeds, Photo Technique, 1, 12 (1939). 



