EXPOSURE AND EXPOSURE DEVICES 



257 



Characteristics of Photoelectric Exposure Meters. — Table XXVI gives the essential 

 operating characteristics of the more important photoelectric exposure meters avail- 

 able in the United States.^ 



Desirable characteristics of exposure meters of this type include: (1) approximate 

 equality of horizontal and vertical acceptance angles, (2) long scale, (3) high sensi- 

 tivity, (4) accommodation to wide range of /-numbers, (5) accommodation to wide 

 range of film speeds, (6) accommodation to wide range of shutter speeds, and (7) scale 

 calibrated in arbitrary units which are proportional to light intensity or, preferably, in 

 terms of photometric units. 



The polar-sensitivity characteristics of a number of photoelectric exposure meters 

 are shown in Fig. 9. 



20 10 10 20 



Fig. 9. — Polar diagram showing the relative sensitivity of a number of photoelectric 

 exposure meters for light coming from the angles indicated. The zero angle is, of course, 

 the optical axis of the meter. The letters refer to the various exposure meters as follows: 



B, Bewi; E, Electrophot (Rhamstine) ; G, General Electric; H, Helios; M, Meltrovick; MP, Mini- 

 Photoscop; P, Photar; S, Sixtus; Wi, Weston Universal 650; Wi, Weston Junior 850; Wi, Weston 

 Cine 819. 



The significance of the subscripts is given as follows: 



h, horizontal plane; vf, vertical plane, foreground; vs, vertical plane, sky. 



Methods of Using Photoelectric Exposure Meters. Brightness-range Method. — 

 Fundamentally we desire to expose the negative, so that the light intensities from the 

 various areas of the scene being photographed will make use of the straight-line portion 

 of the D-logio E characteristic. This means that the exposure of both the brightest 

 part as well as the darkest part of the scene will fall on the linear region of the D-logio E 

 curve. The range between the lightest and darkest light values of the scene is termed 

 the brightness range, and if this does not exceed about 125 {i.e., if the brightest 

 area is not more than 125 times as bright as the darkest area), it is usually possible 

 to make an exposure on modern films so that the straight-line portion of the curve 

 is used. The method by which this is accomplished is known as the brightness-range 

 method. 



To use this method, the readings of the meter should be numerically proportional 

 to the light intensity reaching the meter, rather than in /-stops or arbitrary letters or 

 figures. The light refiected from the brightest portion of the scene as well as that from 

 the darkest portion of the scene is independently measured. The ratio of these two 

 readings determines the brightness range of the scene being photographed. The 

 geometric mean of these two readings is then taken as the light reading for which the 

 exposure should be made. An example will make this clear. 



1 A more complete table of characteristics of photoelectric exposure meters appears in Photo Technique, 

 1, No. 1, June, 1939. 



