74 



HANDBOOK OF PHOTOGRAPHY 



A typical 8- by 10-in. studio camera has the following characteristics: triple exten- 

 sion bellows (30 in.); back capable of being raised to form an angle of 45° with the 

 camera bed or of being dropped through an arc of more than 70°; rising and falling 

 front movement of 3^^ in. above center and % in. below center; lateral movement of 

 lens of 2 in. and a horizontal swing of 80°; weight, 9% lb.; dimensions folded, 4J^ by 

 IIM by UH in. 



Reflex Cameras. — In the reflex camera it is possible to focus the image of the 

 subject to be photographed until the instant of exposure. This important and useful 

 feature is accomplished in the following manner. 



A mirror at an angle of about 45° is placed between the lens and the ground-glass 

 focusing screen which is placed, not directly behind the lens as in other cameras, 

 but in the top of the camera and at 90° to the path of the rays from the lens to the 



sensitive material. The image, therefore, 

 is seen right side up and full size (usually), 

 although reversed from left to right. When 

 the exposure is to be made, the mirror 

 moves out of the way of the light rays. The 

 mirror is pivoted about its upper end, and 

 before the shutter is opened for the expo- 

 sure, the mirror is urged upward by a spring 

 to close the top of the camera so that no 

 light through the ground glass can fog the 

 film. 



The great advantage of the reflex camera 

 is the fact that a moving object can be 

 followed on the ground-glass screen and kept 

 in continuous focus until the exposure is 

 made. For this reason the reflex is the 

 favored camera for action photography. 

 The disadvantages of the reflex are: (1) bulk 

 and weight; (2) to get sufficient light on the 

 screen, it is necessarj^ to focus with the lens 

 wide open. If, then, the lens is to be stopped 

 down before making the exposure, the time 

 taken to adjust the stop and to recompose 

 the picture may eliminate one of the reflex camera's great advantages. 



The ground-glass screen is surrounded by a hood to keep light from it, except that 

 coming from the lens. In small reflex cameras, focusing becomes more difficult 

 because of stray light getting on the screen. Certain of the small reflex cameras have 

 rather large hoods, which keep stray light from the screen and thereby aid the user to 

 get sharp focus. Many small reflex cameras have built-in magnifying lenses located 

 over the center of the screen so that still sharper focus is possible. The grain of the 

 ground-glass screen may sometimes be too coarse to allow the accuracy of focusing 

 necessary when the lens aperture is large. 



One way to get around the difficulty of focusing the reflex camera at small lens 

 apertures is to use two lenses, one for making the picture and one for focusing. These 

 lenses are moved with the same focusing adjustment, so that, when the image is 

 accurately focused upon the viewing screen, it is also in sharp focus on the sensitive 

 material. The focusing lens may have longer focal length than the picture-taking 

 lens, so that an enlarged image will be seen on the focusing screen. Because the 

 depth of field of the focusing lens is shallower than that of the taking lens, the user 

 will get sharper images than if the two lenses have the same focal length. One 



Fig. 5. — Grafiex form of reflex camera. 

 1, hood; 2, ground glass; 3, mirror; 4, 

 focusing knob; 5, lens. 



