EXPOSURE AND EXPOSURE DEVICES 



215 



the various film-speed numbers, as given in Appendix B, although such relations are to 

 be regarded as being, at best, rough approximations. If Sa is some arbitrary or rela- 

 tive film-speed or film-sensitivity number (as given in column 1 of Appendix B) and 

 Sn is the film speed or sensitivity in the n system of rating, then the various film-speed 

 numbers will be related to the relative sensitivity by the equations given in Table 1. 



Table I. — Equations for Various Film-speed Relations 



Film-speed system 



Weston 



American H and D {k — 10) 



European H and D 



Watkins 



Wynne 



Burroughs- Wellcome 



American Scheiner 



European Scheiner ■ . ■ 



Eder Hecht 



DIN 



Eastman* 



* The film-speed values for the Eastman Kodak Co. minimum-gradient film-speed system are based 

 on published data for only a few emulsions, from which it was determined that the Kodak numbers are 

 approximately five times the Weston speed numbers for the same emulsion. 



For those film-speed systems (designated by a Kn factor in the last column) for 

 which the speed numbers are proportional to the relative film speed, the exposure and 

 film speed bear a very simple and convenient relation to one another. To show this, 

 suppose we photograph a very simple object without any detail whatsoever, such as a 

 uniformly lighted sheet of paper. The exposed and developed negative will then have 

 a uniform density, Do, or a uniform opacity, 0. Let the light intensity at which the 

 photograph was made be I, the shutter speed be to, and the film speed be Sno'y 

 consequently the exposure is Eo = loto- Then, if the negative is properly exposed, the 

 opacity will be 



= CSn,E, = C5„o7o<o (10) 



Suppose, now, that we change the film speed and desire the same opacity of the nega- 

 tive. It; will then be necessary to alter the exposure, E, and if 7o remains constant, io 

 will have to be changed to t. Thus, for a new film speed, *S„, we have 



O = CSnE = CSJt 

 and since the opacitj'- is the same in both cases we have that 



Sn,U = Snt 



and thus 



t = 



Sn 



(11) 



(12) 

 (13) 



which shows that for a given photographic effect, the shutter speed is inversely pro- 

 portional to the film speed. As an example of the use of this equation, suppose we 

 were to give an exposure of J^o sec. to a film having a speed of Weston 12. This 

 exposure will produce a negative of some definite opacity 0. If we now desire to 



