The Telephotographic Lens and its Uses 73 



by the exercise of great care, it can be done and 

 some very excellent results obtained. It has even 

 been used on flying birds; but the negatives, in 

 such an instance, are bound to be so badly 

 underexposed on account of the necessarily rapid 

 exposure that must be made that, even by the ex- 

 ercise of the greatest care in the development, 

 they are seldom all that could be desired. 



The stops on a telephoto lens act in directly 

 the opposite manner than do those on an ordinary 

 one, for while, in an ordinary lens, the smaller the 

 stop used the greater the covering power obtained, 

 with a telephoto the reverse is true, and the more 

 it is stopped down the smaller will be the covering 

 power, until, if the smallest stop is used, only a 

 circle in the centre of the plate will be exposed 

 unless the lens is made to cover a much larger 

 plate than the one it is being worked with. So it 

 is always best to use it wide open, or at its largest 

 aperture, and obtain sharpness of definition by 

 accurate focussing. This not only gives better 

 covering power, but also, naturally, greater speed. 



In selecting a shutter for use with this lens one 

 which works as smoothly and with as little jar 

 as possible is desirable, for when an exposure of 

 one or two seconds is required, the jar made by 

 the opening of the shutter will sometimes cause 

 a sufficient vibration to blur the image. 



The usefulness of a telephoto lens lies entirely 



