HOW ARE PHOTOGRAPHS MADE? 145 



over a second is needed. The shutter and diaphragm are 

 controls for the amount of light which is allowed to pass 

 through the lens to produce the image on the film. 



Lenses. If you can get two lenses of the same diame- 

 ter but one thicker through the center than the other, you 

 will be able to make an interesting experiment. If you 

 are in a partly darkened room, light a lamp and hold the 

 thick lens five or six feet from the lamp. Move a sheet 

 of white paper back and forth on the side of the lens away 

 from the light. You will soon find a place where a sharp 

 image of the lamp appears on the paper. Now if you 

 place another lamp a foot forward or back of the first and 

 keep the paper still, you will see that objects at different 

 distances give a fairly sharp image. If you repeat this 

 with the thin lens, you will not be able to produce a sharp 

 image of the two lamps when they are very far apart. 

 The thick lens is a short-focus lens, and this is the type 

 used in the box camera. You do not have to change the 

 position of the lens or "focus" the camera because this is 

 a " fixed focus" lens, that is, all objects in front of the 

 lens and more than six feet distant will produce an image 

 which is fairly sharp. Other cameras the focusing type 

 -use the long focus lens. For photographing near-by 

 objects with such a lens the distance from the film must 

 be adjusted according to the distance scale on the camera, 

 but all objects over 100 feet away will give a sharp image 

 when the lens is set at the 100 mark on the scale. 



What Makes Some Cameras Expensive? You may 

 have wondered why some people pay $50 to $100 for a 

 special lens for a camera. They are paying for the speed 

 of the lens. Only the central portion of the cheap lens 

 can be used to give a sharp image. If the diaphragm is 

 closed so that light enters through the center only, a longer 

 time must be used for exposing the plate. The expensive 

 lens is ground so carefully that the diaphragm may be 



H. & W. SCI. I 11 



