218 HANDBOOK OF PHOTOGRAPHY 



In considering the reflection of light from objects, attention should be directed not 

 only to their reflection factor as determined by visual brightness but also to their 

 reflection characteristics as related to the spectral sensitivity of the film and the 

 spectral characteristics of the light source being used. A white light source has 

 approximately equal energy in all parts of the visible spectrum and, for practical pur- 

 poses, maj^ also be considered to have equal energy in all parts of the spectrum to 

 which the common photographic materials are sensitive. A white object photo- 

 graphed with white light will reflect much of the light into the lens, and the required 

 exposure will be relatively short. 



On the other hand a colored object will reflect that part of the white light corre- 

 sponding to the color of the object and will absorb, more or less, colors of other por- 

 tions of the spectrum. For objects having the same apparent brightness or reflection 

 the photographic effect will be greatest for those colors to which the photographic 

 material is most sensitive — in most cases, the blue end of the spectrum. Thus, for 

 example, of a red and blue object illuminated by white light and appearing equally 

 bright, the blue object will require the less exposure to produce a given density on the 

 film. The reason for this is that the film is, most probably, more sensitive to blue than 

 to red portions of the spectrum. This is certainly true in the case of "ordinary" or 

 orthonon materials whose sensitivity lies almost entirely in the blue region of the 

 spectrum; it is less true for orthochromatic materials which are sensitive to the blue, 

 green, and yellow. While panchromatic emulsions are sensitive to all visible colors, 

 maximum sensitivity is in the blue and green portions of the spectrum, although 

 they have good sensitivity to the yellow, orange, and red. For practical purposes, 

 the visual brightness of the subject may be used as an indication of the photographic 

 effect, especially since the majority of scenes contain objects or areas of neutral colors, 

 or colors having a wide range of spectral values. For fully panchromatic materials 

 which are sensitive to all visible colors, the visual brightness of the object may be 

 taken, for practical purposes, as an indication of the photographic effect. 



The color of the light source must be considered in a similar way. A white object 

 illuminated by a blue light can only reflect blue light to the film. The required 

 exposure may be less than that of a white-light source of equal energy and of a white 

 object. A blue object photographed by a blue light maj^ produce approximately 

 the same photographic effect as the white object illuminated by blue light of equal 

 energy, because in each case the amount of blue light reflected is approximately the 

 same. But a red object illuminated by blue Ught will absorb most of the light, 

 reflecting little to the film, and in this case a comparatively long exposure must be 

 given to produce a negative of specified density. It may happen that no matter how 

 great an exposure is given, the object cannot be successfully photographed vmder these 

 conditions. The rules by which the spectral characteristics of the film, light source, 

 object, and filter (if one is used) affect exposure can be stated mathematically ^ but 

 need not be considered in a practical treatment of the subject of exposure. 



Broadly speaking, the more intense is the light by which an object is photographed, 

 the greater will be the exposure and the resulting density, all other factors remaining 

 unchanged. If a given or specified exposure is desired and the intensity of the light 

 is increased, then the shutter speed will have to be increased, the aperture will have to 

 be decreased (larger /-number), or some other factor will have to be altered in order to 

 compensate for the increase in light. There are some exceptions to this general state- 

 ment, however. A certain minimum amount of light must strike the film before the 

 latent image is impressed on the emulsion; if less than this exposure is given, the grains 

 of silver halide are not altered in the process of development. On the other hand, for 



1 Portions of this subject are treated in greater detail in the chapter on Light Filters. 



