258 HANDBOOK OF PHOTOGRAPHY 



Suppose the brightest part of the scene measures 500 candles per sq. ft. and the 

 darkest part measures 5 candles per sq. ft. The geometric mean of these readings is 

 the square root of their product, or VS X 500 = -\/2500 = 50. Thus the reading 

 50 should be taken as the most suitable light value for which to determine the exposure 

 time for the given aperture and film. On some meters the geometric mean of the two 

 readings maj^ be determined by properly marked scales and Umiting marks, as in the 

 Weston Model 650 Universal exposure meter. 



Darkest Object Method. — Since the range of scene brightness does not often exceed 

 about 128 (which particular figure is chosen because only an approximate value is 

 required and this one is a power of 2), it is often quite satisfactory to measure the 

 brightness of the darkest object or the darkest shadow in which detail is desired, if 

 these can be approached sufficiently closely to be measured and are not below the 

 sensitivity of the exposure meter. This light reading may then be multiplied by 10, 

 and this reading can be taken as the best compromise for determining the proper 

 exposure for the entire scene. 



For example, suppose that the darkest shadow measured 5 candles per sq. ft. as 

 before. The proper exposure for the entire scene, according to the darkest object 

 method would be 5 X 10 = 50, which should be used for determining the shutter speed 

 or aperture from the calibration of the meter. 



Brightest Object Method. — Sometimes the sensitivity of exposure meters is not 

 sufficiently great to permit an accurate reading to be obtained by the "darkest object 

 method." In such cases the brightest object method may be used, which is similar to 

 that just described. In the brightest object method, the brightest object in the scene 

 is measured. The reading thus obtained is divided by 10 to obtain the best meter 

 reading for which to determine the exposure. 



Suppose, for example, that a sheet of paper happens to be the brightest part in the 

 scene being photographed, and that the meter reading is 650 foot-candles. The meter 

 reading of 65 may then be taken as that most suitable for determining the camera 

 adjustments by means of the tables attached to the meter. 



Both the brightest object and the darkest object methods depend upon the assump- 

 tion — which is usually true — ^that the brightness ratio of the scene does not greatly 

 exceed about 100 to 1. If the brightness range is much greater than this, the latitude 

 of the film may be inadequate to permit exposure of both the bright and the dark 

 objects simultaneously on the linear region of the H and D curve. In such cases a 

 compromise must be made if correct exposure is to be obtained. It is then expedient 

 to determine which portion of the scene (light or dark) is of greater interest and to 

 ascertain that the more important part is properlj^ exposed, at the expense of the less 

 important portions. Thus, for example, if a bright sheet of paper is the dominant 

 subject in the entire scene, the brightest object method would ordinarily be the pre- 

 ferred method of the two. If a dark suit is to be properly photographed in the scene, 

 the darkest object method is to be preferred. 



Average-brightness Method. — Where it is inconvenient or impossible to measure the 

 light value of the darkest object directly, then the average values of the entire scene 

 can be measured by directing the meter toward the center of the scene. In this 

 measurement it is preferable to have as little sky included as possible. For example, if 

 buildings with reasonable foreground are to be photographed with open sky overhead, 

 then the meter should be directed slightly downward, so that the imaginary 60° cone 

 area covered by the electric eye does not extend into the skj- area. 



Substitution Method. — When the brightness of a dark-colored object in the shade is 

 very low so that it cannot be measured with accuracy on the instrument, or possibly 

 not at all, its approximate value may often be determined by the following substitution 



