66 Wild Birds 



equivalent focus 8 inches, speed ; Extra Rapid Universal 

 Lens, Series D, g T V inch., speed ^; Plastigmat, No. 4, gi inch., 

 speed ^j. 1 



The convertible anastigmats are convertible in two or three 

 lenses of different foci, according as the single anastigmats are 

 of equal or different foci. They thus combine in a single lens 

 the possibilities of working with short and long foci, the 

 greatest speed being obtained when each system of the doublet 

 has the same focus. 



The cheaper plastigmat lenses are equally serviceable for 

 work in the field. In these the rear lens, which has twice the 

 focal length of the doublet, may be used separately, but of 

 course with a greatly reduced speed. Those who have worked 

 also with the Goertz lenses of similar focus and speed will find 

 little to be desired in the possibilities of the best photographic 

 instruments. 



In photographing animals close at hand, the anastigmatic 

 qualities of a lens count for little. It is depth of focus com- 

 bined with high speed which are most needed, consequently any 

 lens possessing these qualities will answer. 



One of the most difficult problems in bird-photography has 

 hitherto been that of approach within "shooting" distance. 

 The control of the nesting site and the use of the tent offer a 

 solution so far as life at the nest is concerned, in at least many 

 species, and the tent in its general use does away with the need 

 of the very long-focus or telephoto lenses. 



In photographing birds sitting, brooding, or standing at the 

 nest there is no difficulty with a lens of speed -5-, which requires 

 2*5- second to fully expose the plate, at a distance of twenty- 

 eight inches with full lens and strong light. With scenes in 

 which the actors are in constant motion, however, we require 

 a much faster lens, which will reduce the exposure to at least 

 F V of a second. But little is gained, however, in attempting to 

 use long-focus lenses at such short distances, since in stopping 

 the lens to ensure the proper depth of focus we are certain to 



1 These lenses are made by the Bausch & Lomb Optical Co., Rochester, 

 N. Y. 



