36 HANDBOOK OF PHOTOGRAPHY 



in the plane of the viewing lens on which the effective outline of the field can be etched 

 or marked in some suitable way. The size of the etched frame bears the same relation 

 to the camera picture as the focal length of the finder lens bears to that of the camera 

 lens. 



For cameras held close to the face, a direct-vision view finder may be used con- 

 sisting of a plano-concave lens cut to a rectangular shape (Fig. 34). The eye. position 

 is often indicated by a small view hole, and the field limits are marked on the lens at 

 the position corresponding to a semifield <t> projected outward from the point E', 

 which is the virtual image of E formed by the concave lens. In using this finder, the 

 eye must be accommodated sufficiently to see the image of distant objects formed by 

 the lens, and in a small compact camera this may be too close for convenient vision. 

 In such a case, a convex lens may be mounted at E, having its focal plane coinciding 

 \vith the image of distant objects formed by the concave lens, i.e., with its posterior 

 focal point. The finder then becomes simply a reversed Galilean telescope. 



Fig. 34. — Diagram showing optical paths in a direct-vision view finder. 



Parallax in View Finders. — Since the view-finder lens is necessarily displaced to 

 one side of the camera lens, the picture seen in the view finder wUl be correct only for 

 objects at one distance, usually infinity. Some cameras therefore contain an arrange- 

 ment which automatically causes the axes of finder and camera to converge correctly 

 while adjusting the focus so as to intersect at the focused object, while others show 

 a series of frames which outline the field seen at various distances, for instance, very 

 distant, 6 ft. and 2 ft. 



Bibliography 



General geometrical optics: 



Hardy and Perrin: "The Principles of Optics," McGraw. 



Martin, L. C: "Applied Optics," Pitman. 



Gleichen, a.: "Theory of Modern Optical Instruments," H.M.S.O. 



Optics of photography: 



Traill Taylor, J.: "The Optics of Photography and Photographic Lenses," Whitaker. 



CoNRADY, A. E.: "Photography as a Scientific Implement," Blackie & Son, Ltd. 



Lummer: "Contributions to Photographic Optics," Macmillan. 



Cole, R. S.: "A Treatise on Photographic Optics," Low. 



Fowler, H. A., and L. E. Varden: Optical Glass in Photography, Am. Phot., November, 1938. 



Lens-testing methods: 



RiCHTEB, Merte, and von Rohr: "Das photographische Objektiv," p. 367, Springer. 



Jewell, L. E.: J. Optical Soc. Am., 2, 51 (1919). 



Bennett, A. H.: Nat. Bur. Standards (U. S.), Bull., 19, 587 (1923); Sci. Paper 494. 



: J. Optical Soc. Am., 14, 235 (1927). 



KiNGSLAKB, R.: J. Optical Soc. Am., 22, 207 (1932). 



Gardner, I. C. and F. A. Case: J. Research Natl. Bur. Standards, 18, 449 (1937); Res. paper 984. 



Williams, R. L.: Testing Sharpness of Photographic Lenses, Am. Phot., June, 1935, 331. 



