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HANDBOOK OF PHOTOGRAPHY 



4. Distance between Lamp and Subject. — If lamps are used at varying distances from the subject, 

 the exposure time for each lamp, separately, should be determined. The resultant exposure may then 

 be calculated by determining the reciprocal of the sum of the reciprocals of the separate exposures. 

 For example, if one No. 1 lamp is 4 ft. from the subject, and one No. 2 lamp is 6 ft. from the subject, 

 the exposures at //16 are J^ and Ji sec, respectively. The resultant exposure time is then 



htt 



Vo-XH 



ti + u yi + Vi 



^0 1 «». 



It should be noted that the resultant exposure time will always be less than the shortest exposure for 

 any given single lamp. 



5. Reflectors. — No very specific data can be given for those cases in which reflectors are used, 

 because the shape and surface of the reflector have large effects on the light beam which cannot be 

 determined unless the reflecting system is fairly completely specified. In such cases it is best to deter- 

 mine the proper exposure by means of an exposure meter. However, as a rough and approximate 

 guide, the exposures given in Table XI may be considered as being approximately correct for the types of 

 reflectors ordinarily used. This statement is based on experience and empirical relations rather than 

 scientific facts and applies to the majority of reflectors for photographic use. It does not apply to 

 spotlight reflectors where the rays are collimated into a very narrow beam, in which case a shorter 

 exposure may be appropriate. 



Table XII. — Exposure for Black-and-white Still Photography, 100-Watt 



Mazda Lamps 

 (Weston Ratings— 10 to 20) 



