The Telepbotograpbic Lens and its Uses 71 



element very greatly reduces the speed. The 

 negative lens is useless by itself and can, therefore, 

 be used only in conjunction with the ordinary, or 

 positive, element. 



The telephoto lens is focussed by altering the 

 distance between the front, or positive, lens and 

 the back, or negative, lens. This is done by 

 means of a thumb-screw on the barrel of the lens. 

 The shorter the camera extension the greater must 

 be the separation of the positive and negative 

 elements, and vice versa. In focussing one must 

 use extreme care, for the plane of sharpness is 

 limited, and therefore the focus must be exact or 

 the resulting picture will be blurred. 



In order to secure as great a range of focal 

 lengths (and consequent magnifications of the size 

 of the image) as possible, the camera used should 

 be capable of a considerable amount of extension, 

 at least twenty to twenty-five inches in a five by 

 seven and correspondingly more in the larger 

 sizes. A telephoto lens will then give enlarge- 

 ments over the ordinary lens of anywhere from 

 three to twelve or fifteen diameters. Of course, 

 the longer the extension of the bellows the greater 

 the resulting enlargement, but there is a conse- 

 quent diminishing of speed ; therefore we are often 

 forced to sacrifice size of the image where speed 

 is absolutely necessary. 



The best telephoto lenses are capable of work- 



