CAMERAS 89 



is not so important as the possibility of separating the wedges from the base prism. 

 The wedges can be placed on the actual lens mounting, while the base prism and eye- 

 piece remain in the camera body. In this way, those portions of the meter which are 

 most sensitive to mechanical shock can be safely protected by the body, thus affording 

 a very high degree of security from breakdown. 



"A still further step in the development of the distance meter may be taken. The 

 distance meter may work on the 'swing-wedge' method, and its field of view is arranged 

 to agree with that of the camera lens. A combined distance meter and view finder 

 thus results. The swing-wedge principle involves the use of two cylindrical lenses, 

 their outer sides plane and their inner sides ground circular, placed in close con- 

 tact. The front lens is concave, and remains stationary, while the rear lens, which is 

 convex, swings from side to side. The combined distance-meter-view-finder makes it 

 possible to increase the field of view through the distance meter that would otherwise 

 only be possible with the rotating wedge distance meter by increasing considerably 

 the size of the camera. Such combination is clearly useful from the point of view of 

 ease in focusing, quickness of exposure after focusing, and certainty of sharp pictures, 

 since there is only one eyepiece to be looked through instead of two." 



Accuracy of Coupled Range Finders. — The following data are taken from a paper 

 by Cornog.^ 



"The principle of the range finder may be discussed in connection with Fig. 17. 

 The 'range' of the object is the distance CO, or R, measured from the center line 

 connecting the two mirrors G and C, and the 'base' B of the instrument is the distance 

 between the centers of these same mirrors. The base B subtends the angle X at the 

 object, so that 



tan X = I ■ (1) 



where X is expressed in radians and B and R in feet, or meters. This relation may be 

 expressed in terms of the position of the lever arm on the scale S (Fig. 17), as in Eq. 2, 



S =^ tan-i I (2) 



"The range of the object is given, therefore, by the expression 



R = r^ (3) 



tan X 



which may be called the 'law' of the range finder. 



"Since the angle X is always very small, it is necessary that a range finder be well 

 constructed if precise results are to be obtained: given a good instrument, the deter- 

 mining factor then becomes the adjustment for coincidence. In a reasonably well 

 constructed instrument of the type of Fig. 17, such as may be found on a camera, the 

 base length is about two inches, and with ordinary care in setting for coincidence an 

 object twenty feet distant can be located within ±4 inches, or within a length of 

 8 inches, while if the object is only three feet away its position can be located within 

 ±0.1 inch, or within a length of 0.2 inch." 



A good lens operating at an aperture of //1. 5 focused on an object 20 ft. distant 

 will have a depth of focus from 17 ft. 10 in. to 22 ft. 10 in.; focused on an object 4 ft. 

 distant objects between 3 ft. 11 in. and 4 ft. 1 J^ in. will be in focus (circle of confusion 

 3^00 in.). Thus it may be seen that the accuracy of adjustment of the range finder 

 is such that the depth of field of even the fastest lens will take care of minor errors in 

 operating the range finder. 



1 Ibid. 



