314 



OUR PHYSICAL WORLD 



ing the ground glass swings vertically and horizontally so that 

 one can, with these adjustments, bring all parts of the object 

 into focus at the same time. If the object is still, diaphragm 

 down to F.32 or F.64 and find what exposure must be given with 

 such openings by means of the exposure meter. If the object 

 is moving, decide how rapid the exposure must be. The nearer 



FIG. 159. Front of camera lens to show device for setting the time (above) 

 and the diaphragm. Shutter release is at left. 



you are to a moving object, the more rapid its apparent move- 

 ment will be in the image. It might require an exposure of one 

 one-thousandth of a second to catch an unblurred image of a 

 running athlete, while a more distant tree whose branches 

 were swaying in the wind would need only one twenty-fifth 

 of a second. Having decided on the time of exposure, consult 

 the exposure meter for the size of diaphragm opening to be 

 used. Set the diaphragm and the timing device (Fig. 159). 



