254 HANDBOOK OF PHOTOGRAPHY 



meter appears to be limited only by physical damage. By means of a calculator or 

 tables the meter readings can be translated into exposure values. When a photo- 

 electric exposure meter is directed at a scene its indication depends upon the average 

 brightness of the area taken in by the meter. The angle subtended by the diameter 

 of this area, having its apex at the exposure meter, corresponds to the view angle of a 

 camera and is usually referred to as the acceptance angle of the exposure meter. This 

 acceptance angle is governed by the location of the cell with respect to the case or by 

 the use of baffles over the photoelectric cell. 



Operation of Photoelectric Exposure Meters. — It is a characteristic of photoelectric 

 exposure meters, as well as of other types of exposure devices, that the integrated or 

 "average" illumination upon the subject is the quantity which is determined by the 

 instrument. But from the photographic aspect the subject is composed of areas of 

 different color and luminous intensity, and it is these differences in intensity (and in 

 color photography, the differences in color) which it is desired to record on the photo- 

 graphic plate. Since the subject maj^ consist of varying areas of dark and light sub- 

 jects, it becomes evident that the integrated light may not result in the best exposure 

 value. This may be made more clear by means of an example. 



Suppose we have a checkerboard of alternate white and black squares which we 

 wish to photograph in such a manner as to reproduce most nearly the original image. 

 From the discussion on page 204 of the chapter on Photographic Sensitometry, it is 

 evident that the exposure should be such that light reflected from the white areas 

 should provide an exposure for the negative falling on the straight-line portion of the 

 D-logio E curve. Similarly the light reflected from the black squares should be such 

 as to produce an exposure which will fall upon the straight-line portion of this same 

 characteristic. When an exposure meter is used to determine the exposure, the meter 

 will read a value of illumination (as a result of light reflected from the checkerboard) 

 which is the integrated sum of the white and black areas. A single reading rather 

 than two (one for the black squares and one for the white squares) is obtained, much 

 the same as if the entire cardboard were gray in tone. Of course, this integrated meter 

 reading serves some purpose since it tends to prevent gross over- or under-exposure, 

 but the method of measuring average light or obtaining an average exposure value is 

 not always a certain method of obtaining correct exposure data (see page 257 which 

 explains methods of using exposure meters). 



Two inherent difficulties have been encountered in the above example, and each 

 may be overcome with intelligent use of the exposure meter. These have to do with: 

 (1) the angle of view throughout which the photoelectric exposure meter is effective, 

 and (2) the fact that the indication of the meter is due to the average brightness of the 

 object or scene included within the acceptance angle of the meter. 



The acceptance angles of most photoelectric exposure meters on the market at 

 the present time are too large, as many of them exceed the angle of view of the typical 

 camera lens, which is approximately 60°. This means that all the light within a cone 

 whose angle is 60° ( ± 30° from the axis) has an effect upon the photoelectric cell, but 

 because of reflectance of light by the glass over the cell and the surface of the cell, the 

 loss of light at oblique angles is appreciable. Therefore all exposure meters tend to be 

 quite selective to the center part of the scene. Consequently, if it is desired to obtain 

 readings of light or dark areas independently of the integrated light value of the entire 

 scene, it is necessary to make the readings with the exposure meter sufficiently close 

 to the subject so that the desired area to be measured, but no other area, falls within 

 the 60° cone. For example, if the light intensity of a person's face is to be determined, 

 the exposure meter should be pointed at the face and should be not more than about 

 a foot from the face when the reading is taken. The reading obtained will indicate 

 the average value of the light reflected from the face, which may be quite different 

 from that reflected from a dark suit. 



