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PHOTOMICROGRAPHY 



obtain magnified images of small objects with very 

 considerable success, and have the advantage over micro- 

 anastigmats of a much longer distance between subject 

 and lens, combined with a shorter camera extension to 

 give the same magnification. There is, however, the dis- 

 advantage that the area of good definition is not very 

 large, but their special properties make them very suitable 

 for natural history photography, in which a large field 

 is not of the greatest moment. 



An ordinary anastigmat lens can be made into a tele- 

 photo combination by the addition of a negative element 

 of shorter focal length, and many makers supply a suitable 

 negative for use with the photographic lenses. Any 

 defects of the positive lens are magnified if this course 

 is taken, and it is better to use a specially made combina- 

 tion in which the elements have been carefully adjusted 

 the one to the other. 



The magnification obtained depends on the focal length 

 of the combination, on the relative object and image 

 distances, and on the separation of the lens components. 



The focal lengths of the components of the " Adon," 

 a typical telephoto lens, are, positive 4J", negative 2". 

 The separation of the two lenses is adjustable by rack and 

 pinion up to 2J". For photomicrography a larger separa- 

 tion than this is needed, and extra tubes are supplied 

 for the purpose. The following figures give the extensions, 

 etc., required by this lens, compared to a 3" anastigmat. 



