EXPOSURE AND EXPOSURE DEVICES 211 



From this discussion, it is evident that the following conditions must be fulfilled 

 in order that a scientifically correct negative may be produced: 



1. The brightness range of the original object must not exceed the latitude of the 

 photographic material; or, conversely, 



2. The latitude of the photographic material must be at least equal to, and 

 preferably should exceed, the brightness range of the object being photographed. 



3. The more nearly equal are the brightness range of the object and the latitude 

 of the photosensitive material, the more critical is the exposure which must be given to 

 obtain correct tone rendition. 



4. If the brightness range of the object exceeds the latitude of the film, only a 

 portion of the scene will be reproduced with proper tone value; other portions of the 

 object will have tone distortion. 



5. If the brightness range of the object being photographed is less than the latitude 

 of the film any one of several correct exposures may be used. 



6. The greater the ratio of film latitude to brightness range of the object, the less 

 critical need be the exposure of the film. 



Definition of Exposure. — The altering of the silver halide grains by the photo- 

 chemical action of the light in order to produce a latent image is called exposure. 

 To a first approximation^ it has been found that the photochemical action taking 

 place during exposure obeys the reciprocity law of Bunsen and Roscoe. As an 

 application of this law to photochemical reactions, the exposure E may be expressed as 



E = It (1) 



where / = the intensity of the light acting upon the sensitized photographic material; 

 t = the time during which this illumination is permitted to act on the photo- 

 graphic material. 

 For exposure to white light, which represents the usual conditions, the exposure is 

 measured in meter-candle-seconds. As indicated in the chapter on Photographic 

 Sensitometry, the light source has a spectral distribution like that of mean noon 

 sunlight, and the exposure time t is usually continuous rather than the integrated 

 effect of intermittent or chopped exposures.^ The equation shows that the exposure, 

 and consequently the photographic effect as measured by the density of silver deposit, 

 depends directly upon the intensity of the light source and increases the longer the 

 material is subjected to light rays. Because the exposure depends upon the time 

 during which the light acts on the film, the film is able to integrate the quantity of 

 light falling upon it. A practical advantage of this effect is that through sufficientlj^ 

 long exposure it is possible to photograph objects which might otherwise not be 

 sufficiently bright to produce a photographic image. 



The exposure is not the only factor determining the photographic effect produced, 

 although it is a very important factor in this connection. The photographic effect, 

 by which is meant the density of the silver deposit, is determined bj' the characteristics 

 of the sensitive material and by the processing conditions as well as by the exposure. 

 These factors are related graphically by means of the Z)-logio E characteristic and 

 are discussed in the chapter on Photographic Sensitometry. 



By means of the D-logio E characteristic curves, it is possible to determine the 

 density produced on a certain photosensitive material for given exposure and process- 

 ing conditions. Such curves provide a clue to what might be expected, bj^ way of 



1 Careful investigations show that the reciprocity law is not exactly obeyed by photographic mate- 

 rials. Failure of the reciprocity law is not of serious consequence in most branches of practical photog- 

 raphy, and, for a first approximation, may be neglected. 



^ If the film is exposed intermittently, it is found that the photographic effects are not the same as 

 when the film is exposed for the same time duration but continuously rather than intermittently. 



