40 Photography for the Sportsman Naturalist 



The depth of focus of a lens is the distance that 

 the ground glass can be moved to and fro with- 

 out the image of the object upon which the lens 

 is focussed becoming visibly unsharp. This per- 

 missible racking increases correspondingly as the 

 lens is stopped down. This term is often con- 

 fused with depth of field, which is the distance 

 between two objects lying in a straight line with 

 the camera and which are at both extremes of the 

 field of sharp focus. This also increases materially 

 with each smaller stop used. 



The " stops " or " diaphragms " of a lens are 

 used to increase sharpness of detail, depth of field, 

 and depth of focus. These stops are simply a 

 series of apertures of varying sizes, which are 

 either made in a rotating plate, when they are 

 known as " rotating stops " ; or in separate plates 

 which are slipped into the lens mount and are 

 called " Waterhouse diaphragms " after the in- 

 ventor; or, and this is the most usual form, the 

 apertures are formed by a set of thin plates which 

 open and close like the iris of the eye, and which, 

 consequently, are called "iris diaphragms." 



The most practical function of the diaphragm 

 is that, by decreasing the aperture, various planes 

 are brought equally into focus at the same time. 

 This, necessarily, by lessening the illumination 

 of the plate, increases the requisite length of 

 exposure correspondingly with the diminishing 



