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HANDBOOK OF PHOTOGRAPHY 



more natural. The viewer may have to shut one eye so that he can approach close 

 enough to the print to embrace all of it, but the perspective will be more nearly like 

 that made with the long lens. 



If a scene is photographed with a lens covering a very wide angle, it will be difficult 

 to view the entire picture at once, so that the print will be moved away from the eyes. 

 This distorts the perspective. On the other hand, if a telephoto lens is used which 

 covers a very narrow angle, it will be natural for the user to move the print closer to 

 the eyes so that the print fills the field of vision. This distorts the perspective in the 

 other direction. 



The important criterion is that the print is to be viewed at such a distance that the 

 angle subtended at the eye is equal to the angle subtended at the lens by the object. 

 An enlargement from a negative made with a short-focal-length lens and viewed 

 from the proper viewing distance will often have a better perspective than a print 

 made by contact with a negative produced with a lens of longer (although improper) 

 focal length for the viewing distance to be employed. A 5- by 7-in. print made with 



a 7-in. lens should be viewed at 7 in. viewing distance. If 

 held at 10 in. the perspective will be somewhat unnatural. 

 A negative made with a 2-in. lens and enlarged 5 times 

 will produce a print when sdewed at 10 in. which will 

 have more natural perspective than the contact print of 

 the same size made with the 7-in. lens. The difference 

 in perspective, however, will be small. 



If L is the viewing distance of the print, / is the 

 focal length of the taking lens, and n is the linear mag- 

 nification of the print compared with the negative, then^ 



D=Nd 



L =n/ 



(9) 



Fig. 27. — Relations be- 

 tween focal length of taking 

 lens and proper viewing 

 distance for print. 



from which the correct viewing distance or the best linear 

 magnification may be found if the other two factors are 

 known. 



Example. — What is best degree of enlargement for a negative made with a 2-in. (5-cm.) lens when 

 the print is to be viewed at 30 in.? (This figure approaches the distance at which salon prints are 

 viewed.) 



7 30 



" = 7 = T 



= 15 



Thus a print from a 35-mm. negative should be arproximately 13 by 20 in.; or at a viewing distance 

 of 10 in. the best size for a 35-mm. enlargement is 5 by 7 in. 



Choice of Focal Length. — Since the eye includes an angle of about 50°, it is advis- 

 able to include only this angle in a print. Thus we have the rule that the focal length 

 of the lens should be equal approximately to the diagonal of the plate or film to be 

 covered. For example, a certain lens designed to cover a 4- by 5-in. plate has an 

 equivalent focal length of ^^%2 in. Focused upon an object at infinity, this lens 

 subtends at the plate an angle of approximately 53°. 



1 This may be proved by Fig. 27. Let d be the diagonal of the plate to be covered by a lens of focal 

 length /, and let D be the diagonal of the enlargement. The condition is that the print and the negative 

 subtend equal angles at the eye of the observer and at the lens, respectively. 



By similar triangles, D/2 -=- d/2 =L-^f=D-i-d. 

 But D = nd. Therefore 



Nd _L 

 d -f 



N = ^ 



