Cameras, Lenses, and Shutters 41 



of the size of the aperture. The stops of a lens 

 are all numbered, and, speaking very roughly, the 

 time of exposure must be doubled with each 

 smaller stop used. That is to say, if one second 

 is the proper exposure on a subject when the 

 lens is wide open, then, when using the next 

 smaller stop, two seconds must be given, and 

 when the next smaller is used, four seconds, and 

 so on. 



We can see by this that the larger the aperture 

 of a lens the more speed it has, and by this means 

 the speed of a lens is determined ; but, also, with 

 the larger apertures there is a corresponding 

 diminishing of the depth of field. A lens work- 

 ing at F.6 (which means that its largest aperture 

 is F.6) is twice as fast as one working at F.8, 

 but its depth of field is diminished accordingly. 

 However, by stopping it down to F.8 we have the 

 same depth of field and the same speed as we 

 would have with the one working at F.8 used 

 wide open. 



The different systems of marking lenses are too 

 intricate and would take up too much room for 

 me to go into a detailed explanation of them here, 

 nor do I think it at all necessary. Suffice it to 

 say that the F. system is the one most generally 

 used, and it represents the different aperture ratios 

 and is intended to give the operator an idea of 

 the relative speed of his lens when used at the 



