EXPOSURE AND EXPOSURE DEVICES 



237 



Table XII gives recommended exposures for artificially lighted subjects for various apertures, types 

 of reflectors, and distances between lamp and subject. 



1. Film Speed. — Table XII is based on an assumed film speed for tungsten of Weston 10 to 20. For 

 other film speeds the required exposure time is inversely proportional to the Weston rating of this film. 



2. Aperture. — The exposures for various apertures are given, but conversion may be made for other 

 apertures, if this is desirable, by Eq. (23). 



3. Number of Lamps. — Table XII is based on the assumption that one 100-watt lamp is used, having 

 a clear or white-frosted, rather than daylight, bulb. If daylight bulbs are used, the exposures will have 

 to be increased for the same lamp rating, because of the filtering action of the bulb. For lamps of other 

 power ratings than 100 watts, the exposures given should be multiplied by 100/ PF, where W is the power 

 consumption of the lamp used, in watts. 



If several lamps at varying distances from the subject are used, the resultant exposure may be 

 found from the individual exposures required for each lamp. If the exposure time required tor one lamp 

 is <i, and that required by another lamp, ti, the net or resultant exposure is given by 



titi 



h + ti 



4. Subject. — Table XII is based on average- or medium-colored subjects. If dark subjects are 

 photographed, the exposure should be increased. The exposure may be decreased if light-colored 

 objects are being photographed. 



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 deter- 

 mined unless the reflecting system is specified. The values given may be regarded as correct for average 

 conditions, however. 



Table XIII. — Exposures for Still Black-and-white Photography with 



Photoflash Lamps 

 (Weston Ratings— 10 to 20) 



Photoflash 



Type of 

 reflector 



Distance, 



lamp to 



subject, ft. 



No. 10 lamp 



None 



Paper'' 



Metal* 



Aperture 



No. 20 lamp 



None Paper* Metal* 



Aperture 



No. 75 lamp 



None Paper* Metal* 



Aperture 



10 

 15 

 20 

 30 

 50 



//50 



//38 



//30 



//25 



//18 



//15 



//lO 



//8 



//5.0 



//3.0 



//54 



//42 



//35 



//28 



//22 



//14 



//ll 



//7.0 



//4.2 



//50 

 //38 

 //30 



//25 



//19 



//16 



//lO 



//8 



//5 



//3.0 



//54 



//42 



//35 



//27 



//22 



//14 



//ll 



//7.0 



//4.2 



//50 

 //38 

 /.32 

 //20 

 //1 6 

 //lO 

 //6.0 



* See note 3. 



Table XIII gives suggested exposures for black-and-white photography with photoflash lamps with 

 various types of reflectors for various distances between lamp and subject. 



1. Film Speed. — The table is based on a tungsten film speed of Weston 10 to 20. For other film 

 speeds, the required /-number is directly proportional to the Weston rating. 



2. Number of Lamps. — Table XIII is calculated for one photoflash lamp in each reflector. If n 

 similar lamps are used in the same reflector, the /-number given should be multiplied by n. For example, 

 if four No. 10 lamps 10 ft. from the subject are used in a polished-metal reflector, the aperture would be 

 4 X 7.5 = //30. 



3. Reflectors. — Where reflectors are used, the data are to be regarded as approximate because the 

 manner in which the reflector directs the light modifies the exposure considerably. 



