CHAPTER V 



THE TELEPHOTOGRAPHIC LENS AND ITS USES 



The telephoto lens is of such inestimable value 

 to the nature worker, and so few people really 

 give it credit for what it can do, that I think a 

 chapter none too little to devote to it and its 

 uses. 



To define it very untechnically, it is a lens 

 having the properties of a telescope in a dimin- 

 ished degree. Its main characteristics are two : 

 it will give photographs of objects on a larger 

 scale than will an ordinary lens with the same 

 extension of bellows and from the same point of 

 view ; it is not of any fixed focal length and can 

 be focussed to give a sharp image at any camera 

 extension (measured from the negative lens to the 

 ground glass), provided only that the extension is 

 as great as, or greater than, the focal length of the 

 front, or positive, lens. 



The front lens is generally an ordinary photo- 

 graphic lens, and any make can be used for that 

 purpose. The faster lenses are, however, prefer- 

 able, as the addition of the rear, or negative, 



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