EXPOSURE AND EXPOSURE DEVICES 217 



factors represented by letters having zero subscripts are known and the exposure is to 

 be calculated for entirely different conditions. 



Suppose we know from experience in copying a photograph full size (Mo = 1), 

 when the brightness is 100 candles per sq. ft. (/o = 100), the film speed is Weston 16 

 (So = 16), and the aperture stop is //8 (/o = 8), that the shutter speed is }4o sec. 

 (^0 = Mo)- No filter is used for the known conditions so that Fo = I. The trans- 

 mission of the lens will not usually be known, ^ but for the sake of argument, assume a 

 Tessar is used with a transmission of 85 per cent, so that To =^ 0.85. 



Now suppose we wish to determine the shutter speed when we use a film whose 

 speed is Weston 20 {S = 20), an aperture stop of //5.6 (/ = 5.6), a filter whose factor 

 is 4 {F =4). Assume we use two identical lamps, instead of one as previously, so 

 that / = 2/o = 200. Suppose we use a lens with more glass elements than before, 

 so that the transmission may be taken as 68 per cent. Let us copy the photograph 

 double size so that M = 2. 



By multiplying Eq. (17) by t, we find the time exposure to be 



Substituting the values given above, we find 



To ^ yt) ^ (20) ^"^^ (200) ^ L(i + D^^J ^ 



^ ~ 1^ ^ ITJ ^ lisj ^ ^ ^ [m) ^ I ( 1 + li^'J ^ 0.68 " 4o^^''- 



The value found is approximately 1/4.5 sec. so that }i sec. or }i sec. should be used, 

 depending upon the shutter speed available. 



Exposure. Practical Considerations. — In this section, it is proposed to discuss 

 in some detail and from a practical rather than a theoretical point of view, the various 

 factors which enter into and determine the exposure which must be given to the 

 photographic material. Fundamentally, as we have seen from theoretical considera- 

 tions, the important factors affecting exposure include: brightness of the object, 

 aperture stop or /-number, filter factor, shutter speed, film speed or sensitivity, 

 magnification of the image, and transmission of the lens system. From the practical 

 point of view, however, a number of these factors frequently require simultaneous 

 consideration in photographic practice. Thus, for example, the transmission of the 

 lens system, as well as the magnification of the image, is involved when supplementary 

 or telephoto lens are used or when only a single element of a doublet is used. Like- 

 wise the magnification of the image occurs in such practical matters as enlarging or 

 reducing the size of the image or in copying to size. The intensity and quality of the 

 illumination of the object is especially important when photographing outdoor sub- 

 jects by daylight, as the time of day, condition of the sky, and type of subject being 

 photographed influence the intrinsic brightness of the object. 



Intensity and Quality of Light. — The intensity and quality of the light reaching 

 the film from the object is undoubtedly the most important consideration in deter- 

 mining the photographic exposure. In comparatively few cases, as in astronomical 

 photography, the objects being photographed are self-luminous. In this case only 

 the intensity and spectral distribution of the luminous source need be considered. 

 But where objects are illuminated by reflected light, as occurs in the great majority of 

 instances of practical photography, the magnitude and spectral reflectance of the 

 object must be considered as well as the intensity and quality of the light by which it 

 is illuminated. For bright objects, for which the reflectance is high, more light will 

 be produced at the camera lens from a given source of illumination than for those 

 cases in which the objects are dark and the reflectance is small. 



' See p. 226. 



