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HANDBOOK OF PHOTOGRAPHY 



Principle of Range Finders. — In Fig. 13 consider two rays of light from an object E 

 so far distant that the rays from it are essentially parallel. Let these rays pass through 

 apertures and Q. The rays will also pass through A and B. Now consider another 

 object situated on the ray passing through 0. The ray from D passing through will 

 pass through B but, because of the closeness of D, a ray from it passing through Q will 

 not pass through A but rather through some point such as C. The closer the object 

 D, the further apart will B and C be. Thus the distance between B and C is a measure 

 of the distance of the object D from the line OQ. If some means is provided at Q to 

 bend the ray from D so that it goes through A, the amount of adjustment necessary 

 will be a measure of the distance from to D. A fixed point of comparison is provided 



From 

 \ distant 

 [object 



I ^ 



01 D 



Fig. 13. — Elements of coupled range finder. 



by reflecting the beam twice and moving B over to the original position of A. Thus 

 B and C, when adjusted to C, are presented to the eye together, and any difference 

 in their positions is readily seen (see Fig. 14). 



The distance between and Q is known as the base of the range finder. Increasing 

 the base increases the accuracy of the distance determination but increases the diffi- 

 culty of using the device. If too great a base distance is used, the two images may be 

 so far apart that it will be difficult for the user to find them. 



Kodak Pocket Range Finder. — In Fig. 15, with the eye at the indicated position, 

 the field of view is divided horizontally by the edge of the mirror 1 so that one-half 

 of the field comes straight to the eye while the other half is brought to the eye by 



Fig. 14. — Mechanism of range finder by which point C is moved to C", the relative motion 

 required being a function of the distance. 



reflections from mirrors 1 and 2. These mirrors are approximately Ihz inches apart. 

 This distance constitutes the base of the range finder. When the mirrors are parallel 

 to each other, the vertical lines of an object at infinity appear continuous in the two 

 halves of the field. For objects closer to the range finder, mirror 1 must be moved 

 so that it is not parallel to mirror 2 in order to make the two halves of the field merge. 

 This is accomplished by turning the actuating cam which swings the lever bearing 

 mirror 1. The spring urges the lever against the cam so that backlash in the cam is 

 eliminated. Because of the definite relation which exists between the angle between 

 the mirrors and the distance from the object as viewed in the range finder, it is possible 

 to calculate a scale which indicates the distance directly when the two halves of the 



