THE OPTICS OF PHOTOGRAPHIC LENSES 



27 



The third point to notice is that for proper perspective the print must be viewed 

 from the point originally occupied by the center of the lens. With amateur snap- 

 shots, this point is frequently too close to the print for the eye to accommodate, and 

 a magnifier must be employed. The ideal magnifier is one having a focal length 

 equal to that of the camera lens. If such a magnifier is used, the improvement in 

 perspective is often amazing, and the whole scene takes on an impression of great 

 reality. If the picture is enlarged, the viewing point is carried away from the print in 

 proportion to the degree of enlargement, and for this reason alone enlargements of 

 snapshots are frequently found to be much more "natural" and pleasing to the eye 

 than the original contact prints. A good rule with miniature cameras is to enlarge 

 by the ratio of 15 in. to the focal length of the camera lens, on the assumption that 

 the enlargement wdll be viewed at approximately 15 in. distance. 



A fourth factor which sometimes accounts for an unnatural picture is the size of 

 the angular field of view covered by the photograph. Artists rarely show more than 

 30 or 40° in one picture, and photographs which include very much more than this are 

 often deceptive. Almost all photographs taken with a wide-angle lens covering a 

 field of 80 or 90° or even more are unsatisfactory, a typical example being a close view 

 in a small steamship cabin which looks like a wide and spacious room in the print. 

 The data in Table VIII may be useful, relating the focal length and size of angular 

 field of lens with the size of picture produced by it. 



Table VIII. — Angular Field of Lens 



