Photographic Equipment . 



Photograpliy, with its far-reaching and accurate 

 records, has invaded every department of scientific in- 

 vestigations. Whether it be the farthest star, millions 

 of miles away in unlimited space, or the microscopical 

 death-dealing microbe, this aid to the investigator has 

 made possible permanent records for future workers. 



The improvement made in lenses and shutters in this 

 department of science has reached a perfection thought 

 impossible up to a few years ago. From the single 

 meniscus, concavo-convex lens with all possible errors 

 and slowly working powers, to the single achromatic with 

 its partial chromatic correction, to the rapid rectilinear 

 corrected for most errors except curvature of field and 

 astigmatism and, finally, to the fast-working anastigmat 

 with its full correction for all optical errors, is quite a 

 gap and represents the very climax in lens-making. 



The anastigmat is a fast-working lens, giving critical 

 definition at full apertures of F. 3.5 to F. 6.3, and 

 smaller. Many of these lenses have symmetrical front 

 and back elements and may be used singly at the expense 

 of lengthened focus and much increase in the time of 

 exposure, but with marked increase in the size of the 

 image on the ground glass. In photographing birds 

 rapid working is a most essential quality of any lens, yet 

 depth of field and speed, when near an object, are of 

 equal importance. By depth of field is meant the distance 

 from the nearest object to the farthest in focus. Depth 

 of focus is the distance ground glass may be moved, when 

 the object is in focus, without blur. All lenses, regard- 

 less of make, of the same focal length and the same stop 

 or aperture, are of the same depth of field and focus and 

 of equal speed. The shorter the focus the deeper the 

 field. The longer the focus length the larger the image. 



The speed at which the various lenses work may be 

 stated as follows : 



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