CAMERAS AND PICTURE-MAKING 3 1 1 



object to be photographed is 10 feet away the operator sets the 

 pointer over the lo-foot mark on the scale; if it is 100 feet away 

 or more, over the loo-foot mark. The position of these marks 

 on the scale has been previously determined by the maker of 

 the instrument by focusing on a ground glass in the position 

 later occupied by the film. 



In practically every camera, a diaphragm is provided with 

 openings in it ranging from small to large, so that the photog- 

 rapher can admit through the lens a small amount of light, 

 cutting off most of the marginal rays; or he can use a large 

 opening admitting more light, but using more and more of 

 the marginal rays as the opening is increased in size. The 

 size of the diaphragm opening is usually expressed in terms of 

 the focal length of the lens. Thus when the diaphragm openings 

 are marked F.i6, F.8, F.4.5, the symbols mean that the openings 

 in the diaphragm are one- sixteenth, one-eighth, etc., of the 

 focal length of the lens. This insures that, no matter what the 

 focal lengths of the lenses may be on several cameras, the same 

 sized pencil of light is brought to a focus on the plate when their 

 diaphragms are set for the same opening. In some cameras 

 the diaphragm openings are marked on the universal system 

 (U.S.) in which each larger diaphragm is twice the area of the 

 next smaller size. The U.S.i6 diaphragm is just the same size 

 as the F.i6. From this it follows that U.S.4 equals F.8, U.S.8 

 equals F.n approximately, U.S.i6 equals F.i6, U.S.32 equals 

 F.22 approximately, and U.S. 64 equals F.32. 



It is furthermore evident that much more light enters the 

 camera with a large diaphragm opening than with a small one. 

 In fact, the amount of light varies as the squares of the diameters 

 of the diaphragm openings. An F.8 admits four times as much 

 light as an F.i6. 



Since it is the light that acts upon the plate, the length of 

 time that the plate is exposed must depend on the size of the 

 diaphragm used, the speed of the plate, and the intensity of 

 the light at the time of exposure. The exposure on a bright, 



