CAMERAS 



91 



Image 

 plane 



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

 / = the working aperture //number; 

 C = the diameter of the circle of confusion in inches. 



The hyperfocal distance is sometimes defined as 

 the distance of the nearest object in focus when the 

 lens is focused upon this object sharply and when 

 objects very far away are acceptably sharp. The 

 value of H is used in calculating depth of focus as 

 outlined below. 



In making a depth-of-focus table the distances 

 desired are the distance to which sharpness extends Y\g. 24. 

 beyond and inside the distance upon which the lens focused to a point only in image 

 is focused. Thus, if the lens is focused upon a plane plane. Elsewhere the point 

 10 ft. from the camera, between what limits will other becomes a circle, 

 objects be focused? These distances may be obtained as follows: 



-Point source 



Near distance = -^^ — ; — = Dat 

 H -\- a 



Far distance = v? = J^f 



H — a 



(5) 

 (6) 



and for objects 6 ft. or less from the camera 



Near distance = 



H X a 



Far distance = 



" + {"-1) 



H X a 



H 



(«-/.) 



= D. 



Df 



(7) 



(8) 



where H = the hyperfocal distance in feet; 



a = the distance in feet to which the camera is focused; 

 / = the focal length in inches. 



Example. — Assume a lens of 5-in. focal length, aperture //5, circle of confusion of J-^oo in. diameter. 

 What is the hyperfocal distance and what are the nearest and farthest objects in focus when the lens is 

 focused on an object 25 ft. distant? Hyperfocal distance H = F-/{f X C X 12) ft. 



H 



5 X 5 X 400 10,000 



5 X 12 



60 



= 167 ft. 



Therefore, if the lens is focused upon infinity, objects 167 ft. from the camera and beyond will be in focus. 



H X a 167 X 25 



Near-object distance 

 Far-object distance = 



H + a 

 H X a 



167 -t- 25 



= 21.7 



167X25 27_4 



H - a 167 - 25 

 Therefore objects within a range of 21.7 and 27.4 ft. will be in focus. 



It will be noted that the depth in front of the plane upon which the lens is focused 

 is shallower than the depth behind (farther from the camera) the image plane. At 

 25 ft., an object 3.2 ft. in front of the 25-ft. plane will be in focus; an object 4.4 ft. 

 behind this plane will be in focus. If, therefore, it is desirable to make an object 

 closer than 21 ft. be in focus at the same time an object 25 ft. distant is in focus, 



