CAMERAS 



83 



The following formula may be used in marking out a focusing scale. First deter- 

 mine the setting of the lens for focusing upon a distant (infinity) object. Then the 

 difference between the lens-film distance for this setting and that required to focus 

 accurately upon a nearer object will be found from 



D = 



P 



-f 



where/ = the focal length of the lens in inches; 



s = the distance of object to lens in inches; 

 D = the distance, in inches, of the lens from the infinity position. 



Example. — Design a focusing scale for a 5-in. lens. The lens focuses a distant object sharply when 

 it is 5 in. from the film. An object 25 ft. from the lens will be sharply focused when the lens is moved 

 away from the infinity setting by /V(s — /) in- or 25 -i- [(25 X 12) — 5] or 0.085 in. If the object is 

 only 6 ft. from the lens, the lens must be moved away from the infinity setting 25 -r- (72 — 5) or 0.37 in. 

 from its infinity setting. 



With cameras having lenses of focal lengths of about 5 to 6 in. and having focusing 

 scales parallel to the motion of the lens in focusing, the maximum error in estimating 



Fig. 12A. — Range finder in Fig. 125. — Coupled range 

 focused position. finder in unfocused position. 



Fig. 12C'. — Out-of-focus 

 adjustment of range finder 

 with horizontally split file. 



distance is such that at //8 the lens must be set to within J-^2 in- of the position 

 determined by accurately focusing on a ground glass. If the lens has a shorter focal 

 length and correspondingly greater depth of field, then the error in estimating distance 

 and setting the lens for the estimated distance may be greater. On vest-pocket size 

 and somewhat larger cameras the maximum permissible error is of the order of the 

 distance between the 25- and the 50-ft. marks on the focusing scale. For miniature 

 cameras the error should not amount to more than the interval between the 13- and 

 20-ft. marks. Naturally the error may be greater if the camera lens is operated at a 

 smaller aperture. 



Range Finders. — A valuable accessoiy to any camera is a range finder by which 

 the user is enabled to measure accurately the distance of the object from the lens. 

 The photographer may then set the lens-film distance by means of the focusing scale, 

 or, if the range finder is mechanically connected to the lens in some manner, the act 

 of measuring the distance automatically sets the lens-film distance. 



Range finders are of several types (described below) all depending upon the prin- 

 ciple of presenting to the photographer two images of the object. When the range 

 finder is properlj^ adjusted, the two images merge, and the distance to the object is 

 read from a scale attached. In some range finders the object or field of view is split 

 so that one-half is displaced with respect to the other when the range finder is wrongly 

 adjusted. In others two complete images are visible until the exact adjustment is 

 secured. 



