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HANDBOOK OF PHOTOGRAPHY 



sion of apertures was chosen relative to that of the exposure time. The second 

 exposure meter reaches a constant error for small apertures but also results in relative 

 errors for large apertures, also because of the manner in which the progression of aper- 

 tures was chosen. The third instrument was designed so that the apertures 

 and shutter speeds progressed in proper sequence, and the result is that nowhere does 

 it produce any serious errors due to this cause. Errors of 50 per cent might 

 be expected from inexpensive instruments of this type and would probably produce no 

 great harm for amateur work. 



Table XXV. — Comparison of Some Visual Exposure Meters 



The range of film speeds, apertures, shutter speeds, filter factors, and light condi- 

 tions which visual exposure meters include in their operation, of course, varies with the 

 instrument. Usually, however, they may be expected to have from 10 to 20 graded 

 areas for determining the light value. The film speeds, which are usually marked in 

 Scheiner speeds (since most of these devices are made in Europe), range from Scheiner 

 16 or 18 minimum up to Scheiner 30 or 32 maximum, although with the introduction 

 of faster films, the upper range may be expected to be extended in the future. Shutter 

 speeds usually vary from Kooo or }ioo sec. minimum to about 30 min. to 2 hr. for the 

 maximum exposure time. Filter factors up to 16 may be expected to be taken into 

 account on some of the better designed devices, although in many of the less expensive 

 models no provision is made for taking into account the filter factors. Table XXV 

 shows the range of aperture stop, film speed, and exposure time for several well-known 

 makes of devices. 



