172 HOW TO STUDY BIRDS 



as long as in the latter case. The usual rule Is to 

 buy a lens of the next size larger than the one ordi- 

 narily sold with the camera. A 4x5 camera, such as 

 I have recommended above, will allow the use of the 

 lens doublet and its members designed for the 5x7 

 size; for the latter size the lens for the 6>^x8^ in- 

 strument will be all right. 



A little inquiry will disclose the fact that some 

 lenses cost a great deal. These expensive ones are of 

 the type called anastigmats, or corrected lenses. 

 They allow of a more rapid exposure than the others, 

 and give very sharp detail in the picture, even when 

 used at full opening, without being stopped down. 

 For the general camera, of the type first mentioned, 

 an anastigmat is not essential, though it is a good 

 thing to have one if possible. Any good ordinary 

 commercial lens will do quite well, though it is slower. 

 Much of the work is with timed exposures, at small 

 aperture of the lens, which of itself insures sharp de- 

 tail. To test a lens, focus upon a general view and 

 examine the image on the ground glass. If it Is not 

 clear and sharp to the very corners, the lens will not 

 do. When It comes, however, to rapid snapshots 

 with the reflecting camera, a rapid anastigmat Is very 

 desirable. By purchasing American makes or watch- 

 ing for exchange bargains one may save a good deal 

 of money. 



Some anastigmat lenses which are advertised as ex- 

 tremely *' fast " gain rapidity at the expense of length 

 and depth of focus. The longer the focus the greater 



