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HANDBOOK OF PHOTOGRAPHY 



The two images presented to the eye of the user form a field divided into two 

 halves. With the camera in normal position the lower half of the image as viewed 

 in the eyepiece is the fixed portion of the image. When the camera lens is out of 

 focus, the upper haK of the image is displaced to the right or left. Focusing consists 

 in bringing these two halves of the field into proper relationship so that a unified 

 image results. 



In Fig. 16 the two images created by L and M occur on the face of prism Pi at the 

 common boundary of prisms Pi and P3. The rays from M travel above prism P2 

 while the rays from L travel inside the prism P-i and are reflected at a point just below 

 the other rays. Both rays are then deflected bj^ prism Pz back to form the split image. 

 Prism Pi is an erecting prism to present the image to the eye right side up. The field 



lens collects all the rays forming the image so that 

 they are carried to the eye. The ocular lenses are 

 provided so that the images, which are only a short 

 distance from the eye, may be seen easily. The 

 objectives and oculars actually amount to two small 

 telescopes giving a magnification of 3 X . 



The eyepiece, which carries the ocular lenses, is 

 adjustable along its axis to allow for variations in 

 individual eyesight. The user is instructed to adjust 

 the eyepiece for an object 8 ft. away, a setting which 

 enables images for both distant and near objects to 

 be accommodated by the eye. 



Leica Coupled Range Finder. — ^The principle of 

 the range finder fitted to Leica 35-mm. cameras may 

 be seen from Fig. 17. 



"A ray of light^ from the object at O passes 

 through the window A, through the glass sheet C, 

 which is set at an angle of 45° to line O A, through the 

 window D, and into the eye at E. Another ray of 

 light from the same point on the object passes 

 through window F, is refiected from mirror G, then 

 from the glass sheet C, and finally passes through 

 window D into the eye at E. Thus the eye at E will 

 see two images, a direct one, such as that shown in the 

 large circle in Fig. 12, and one reflected from the 

 mirror G, which is the one shown in the small circle in 

 Fig. 12. The mirror G can be rotated about an axis H bj^ means of lever L, and it is 

 obvious that if this mirror G is rotated to the proper position the ray of light OGC will 

 then emerge from window D along the line OE, and the two images will seem to have 

 become one, or are coincident. Corresponding to this proper position of the mirror G, 

 the end of the lever L indicates on the empirically calibrated scale S the distance OC, 

 which is called the "range" of the object at O." 



Figure 18 shows the range finder as fitted to the cameras. A collar, which is part 

 of each lens available for the Leica camera, fits against a small roller which in turn 

 actuates the prism of the range finder. The field seen in the range finder is enlarged 

 about 13^ times. A circular image is seen in the Leica range finder, the center of 

 the image being brighter than the surrounding part of the field. Two complete 

 images are seen in tliis center portion unless the range finder is properly focused for the 

 desired object. 



Fig. 



17. — Principle of Leica 

 range finder. 



1 CoRNOG, I. Clyde, The Autofocal Camera, Am. Phot., January, 1937, p. 1. 



