TABLE 1 



COMMON F-STOP SYSTEMS AND THEIR 

 RELATIVE ILLUMINATION REDUCIIONS* 



illumination 



U.S. 



English 



Continental 



Reduction 



System 



System 



System 



1 



_ 



1 



- 



2 



- 



1.4 



1.5 



4 



- 



2 



2.2 



8 



- 



2.8 



3.2 



16 



1.25 



4 



4.5 



32 



2 



5.6 



6.3 





2.5 



6.3 



- 



64 



4 



8 



9 



128 



8 



11 



12.7 



256 



16 



16 



18 



512 



32 



22 



25 



1024 



64 



32 



35 



2048 



128 



45 



- 



Another important aspect of photography is the focusing of the 

 camera lens to insure that the objects of interest in the scene to be 

 photographed are in focus; that is, clear and sharp in the resulting 

 picture rather than hazy or blurred. The lens on most simple box cameras 

 is preset so that everything is in focus from about 6 feet to an infinite 

 distance in front of the camera. Therefore focusing is no problem with 

 these cameras. However, on most of the more expensive cameras, the 

 focusing mechanism is adjustable and these cameras must be focused for 

 the objects of interest in the scene before the picture is taken. Some 

 cameras have coupled range finders so that when the proper range is found 

 with the range finder, the lens is automatically focused to the proper 

 distance. Other cameras have uncoupled range finders which simply tell 

 the photographer the distance at which the lens must be set in order to 

 obtain a good picture. When there is no range finder, the distance to 

 the objects of interest must be estimated or measured before the lens is 

 focused. 



In addition to focusing the camera, one must be sure that the range 

 of distances that axe in focus, or the depth of field is adequate to 

 cover all the objects of interest in the scene. The depth of field varies 

 with the properties of the lens being used, the f-stop at which the 

 exposure is made, and the distance from the camera at which the lens is 

 focused. For any particular lens, the depth of field decreases as the 



*Frora Graphic Graf lex Photography , 8th edition, W. D. Morgan and H. M. 

 Lester, New York 17, New York, 1950, p. 25. 



