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HANDBOOK OF PHOTOGRAPHY 



Depth-of-field indicator. 



Depth-of -field Indicator. — A useful accessory included on most miniature cameras 

 and on many larger cameras is a depth-of-field, or depth-of-focus, table. Such a 

 table shows the distance range which will be in sharp focus when the camera is focused 

 upon a given distance and the lens is working at a given aperture. Often the depth- 

 of-field table is in the form of an indicator which shows the user the depth as soon 

 as he has focused the lens upon the desired distance. All such depth-of-field scales 

 or indicators are based upon a certain value of the circle of confusion (see below); 

 on miniature cameras in which the negatives are often enlarged considerably in 

 making prints, the circle of confusion considered is much smaller than is used when 

 large-size images are secured. 



Illustrations of two types of depth-of-field indicators are shown. 



Depth of Field. — If a lens is focused upon a given distance, objects in the plane at 

 that distance will be sharply in focus. Objects near this plane will not be so sharply 

 focused, but they will be sharper than objects at greater distances from this plane. 



If the lens is stopped "down, objects at 

 considerable distance from the plane upon 

 which the lens is focused may be relatively 

 sharp, although not so sharp as objects in the 

 plane upon which the lens is accurately 

 focused. 



The distance from the nearest to the 

 farthest subjects which appear sharply focused 

 is called the depth of field. But this distance 

 range is subjective, depending upon the inability of the human eye to distinguish 

 between a point and a small circle. 



If the lens is focused accurately upon a given plane, then points of light in other 

 planes will not produce points of light in the plane of the film but will produce circles 

 which are cross sections of the pencils of light which come to a point focus in front of, 

 or behind, the film plane. Therefore, the only points of light existing in the film plane 

 are images of points of light in the plane upon which the lens is exactly focused. All 

 other points in front of or behind this plane will be reproduced upon the film plane as 

 circles, larger circles being produced by point sources existing at greater distances 

 from the plane upon which the lens is focused than by points nearer the plane upon 

 which the lens is focused. These circles are known as "circles of confusion." 



It is generally assumed that, if a print is viewed from a distance of 12 in., circles 

 of confusion Hoo in. in diameter or less will not be perceived as circles by the eye; 

 they will look like points. 



A depth-of-field table, can be calculated on the basis of a given circle of confusion. 

 If a camera makes negatives which are not to be enlarged, then the depth-of-field 

 table may be based upon a circle of confusion of K50 in. If, however, negatives 

 are to be enlarged so that the resultant print has circles of confusion no greater than 

 3^50 in., the accuracy with which the camera is focused must be correspondingly 

 greater. Depth-of-field tables used with miniature cameras are often based upon 

 a circle of confusion as small as Mo 00 in. 



Depth-of-field Table. — Assuming a permissible circle of confusion, the distance 

 from the lens of the nearest object which is in focus when the lens is focused upon 

 infinity, is known as the hyperfocal distance. This varies with focal length, circle of 

 confusion, and aperture. The hyperfocal distance, in feet, is 



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