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HANDBOOK OF PHOTOGRAPHY 



the subject so that the filament cannot be seen. By means of scales on the photometer 

 the correct exposure can be determined for various types of films and plates. 



The Zeiss Diaphot exposure disk is an example of the wedge-extinction type of 

 meter. It consists of a rotating disk carrying a continuously variable optical wedge, 

 together with a window for observing shutter speeds. The scales of shutter speeds are 

 contained on a fixed disk which also contains a small eyepiece traveling over the 

 circular wedge. The eyepiece contains a blue filter to destroy sense of color and 

 give visual indication of light intensity only. In operation the photographer looks 

 through the eyepiece to the subject. The wedge is rotated until shadow detail just 

 disappears, when the correct exposure is indicated in the window of the rotating 

 number. 



Fig. 4. Fig. 5. 



Fig. 4. — Leudi visual exposure meter. A series of partially opaque numbers is seen 

 through the slot shown on the upper edge. The exposure is determined in terms of the 

 dimmest number which can be distinguished. 



Fig. 5. — The Instoscope visual exposure meter with focusing eyepiece. 



In the Justaphot and Cinephot visual exposure meters the necessity of measuring 

 the details in the shadow areas is overcome bj'' observing when a translucent figure is 

 just discernible. The figures correspond to exposure time and the light striking them 

 is varied by means of a diaphragm. In operation the figures, corresponding to the 

 desired exposure time, are set in the line of vision in the instrument. The diaphragm 

 is then fully closed and the meter placed in the correct eye position and aimed at the 

 object or scene to be photographed. The diaphragm is then opened until the translu- 

 cent number is just discernible. By means of scales on the meter the correct /-stops 

 for the various films and plates can be determined. 



At the present time the stepped-wedge exposure meters are the most common of 

 the visual type. Many modifications have been produced, but usually this type con- 

 sists of two concentric telescoping tubes. At the end of one of these tubes is a focusing 

 lens and eyepiece which exclude extraneous light when the instrument is held to the 

 light. Near one end of the other tube are several areas of graded density, each of 

 which is either lettered or numbered. The photographer looks through the eyepiece 

 toward the scene to be photographed and sees various graded patches each with its 



