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influence on film speed. However, it should fie stressed that when filters are used, the effective character of the light source is changed. The increase in exposure required by the use of a filter with a given film changes radically with the type of light source also. Such expressions as “a twotimes filter” have no meaning whatsoever. An adjustment not only is required in exposure with the use of filters, but in some cases contrast is aflected greatly enough to require changes from the normal developing time, which, as we have just noted, may have a considerable effect upon film speed.
Under standard conditions of lighting, exposure will be greatly influenced by the character of the subject. Some materials reflect light very highly, whereas others reflect light to a much less degree. The practice based on determining exposure by the strength of the incident light source is consequently fallacious. The intensity of the light incident upon the film is the light which determines the value of the exposure time. Therefore, every consideration should be made to approximate this intensity as closely as possible.
Colored objects, besides differing in reflectivity and brilliance contrast, also vary in color contrast. Under usual circumstances this is not of major importance provided some attention has been given to the photicity response of the emulsion. In some cases, however, when the subject being photographed is predominantly a particular color, this effect may alter exposure from that required for a subject containing a wide color distribution with no predominant hue.
In the foregoing survey, the authors have attempted to make clear that speed numbers, especially as they exist today, are of very limited value. From the many factors upon which the behavior of film emulsions in exposure and development depend, it is quite obvious that the final result obtained on any particular material is considerably dependent on the manner in which it is used. This should not lead one to believe that correct exposure is not important. Correct exposure is exceedingly important, but to obtain it is not nearly so dependent upon the knowledge of absolute speed values as one might he led to believe. As we have shown, no criterion of a correctly exposed negative has ever been arrived at, and so for one to speak of the correct exposure is to speak of something which does not exist. With the wide latitude possessed by photographic materials today, variations in exposure within such wide limits as 3 to 1 give negatives which are hardly distinguishable from each other. If it were not for this fact, “exposure” in practice woidd be an extremely dicult matter, since the errors introduced by the combined factors which we have presented are of a greater order than 3 to 1 . For one to know the speed of a film as a rough approximation is sufficient; for surely, considering the fact that differences in subject and in lighting on any one day may necessitate a range of exposure of several thousand times to one. it is evident that the advantage of knowing the absolute speed value of a film for use in estimating proper exposure is insignificant.
The authors wish to acknowledge the extensive use of the literature
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