PHOTOGRAPHIC OUTFIT 7 



plate. Length of focus is important, in that it gov- 

 erns the size of the object from a given distance. 

 The greater the equivalent length of focus the larger 

 will be the object, and the less will be the distortion 

 due to foreshortening. A short-focus lens fore- 

 shortens all objects that are near with such abrupt- 

 ness as to cause grotesque distortion. The most 

 useful lens for all-round work is one whose two sys- 

 tems or combinations can be used separately. By 

 having such a lens you have practically two in one, 

 the single combination having about double the focal 

 length of the couplet. With some of the newer 

 lenses, such as the plastigmat, the single combinations 

 may be used for instantaneous work, while with some 

 of the other highest grade lenses the single combina- 

 tions can be used only with a small diaphragm. The 

 diaphragm of a lens is used — to give a very untechnical 

 description — to increase the depth of focus and the 

 covering capacity of a lens, thereby giving greater 

 definition at the sacrifice of speed. Each number, 

 such as F8, F 16, etc., means, for practical use, that 

 the exposure when the larger figure is used shall be 

 double that of the preceding figure ; for instance, if, 

 witn the diaphragm set at F 1 6, the exposure needed is 

 one second, at F32 it would require two seconds, 

 and at F64 four seconds. It will be seen by this that 

 for instantaneous work the lens must be used either 

 open or with a large diaphragm. 



