EXPOSURE AND EXPOSURE DEVICES 



225 



25 cm., then the maximum /-number will be / 

 will be 



25/3.33 = 7.7 cm. The exposure 



E 



m 



Eo = 1.662^0 = 2.76^0 



(37) 



or two and three-fourths that of the exposure without the supplementary lens. If a 

 supplementary lens were used which decreased the focal length of the combination, the 

 /-number would be reduced and the exposure with the supplementary lens would be 

 less than that required without it. 



In order to reduce to the minimum the mental labor involved in calculating expo- 

 sures when making enlarged or reduced photographic images, Table IV has been 

 prepared. This table shows the increase in exposure which is required for various 

 magnifications from Koo to 100 in column 2. In the third column, the ratio of the 

 increase in exposure to that required for copying natural size is given. 



Exposure with Single Elements of Lenses. — When a single element of a doublet is 

 used rather than both elements (as in convertible lenses), the focal length of the single 

 element is increased beyond that of the complete lens. So far as the exposure is 

 concerned, the situation is very nearly the same as if a supplementary lens were used 

 to increase the equivalent focal length. Consequently the remarks made under the 

 paragraph above apply equally well here. 



Exposure with Telephoto Lenses. — The function of a telephoto lens is to produce 

 large images of distant objects with relatively short bellows extensions. A telephoto 



