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HANDBOOK OF PHOTOGRAPHY 



insensitive). For orthonon or noncolor-sensitive materials which have a peak of 

 sensitivity at the blue end of the spectrum, it may be impossible to make photographs 

 early in the morning or late in the afternoon except by means of excessively long 

 exposure. 



Moonlight. — Since the light from the moon comes to the earth as light reflected 

 from the sun, it follows that the intensity of moonlight is very much weaker than that 

 of sunlight. Moonlight varies cyclically every 28 days, as well as throughout the 

 night and during the year. As a source of luminous intensity for photographic pur- 



Table I. — Solar Illumination at Normal Incidence. Latitude 42°N. East of 

 Mississippi River. Cloudless Sky^ 



1 From H. H. Kimball, Trans. Ilium. Eng. Soc. (N. Y.), 18, 457 (1923). 



poses, the moon is not very effective. Approximately 150,000 times as long an expo- 

 sure must be given for photographs taken in full moonlight on a clear night as would 

 be required for the same scene by full, direct sunlight. This approximate rule varies 

 with conditions, of course, but a simple practical rule to follow is to expose 25 min. by 

 full moonlight for every Hoo sec. exposure which would be required for the same scene 

 in full sunlight. If the moon is in one of its increasing or decreasing phases instead 

 of being full, the exposure must be still further increased. Of course, if the moon itself 

 is to be photographed, shorter exposures should be given. 



Light from Stars and Planets. — The stars and planets provide so little light as to be 

 suitable only for astronomical photographs, and even here the exposure is usually of the 

 order of minutes, rather than small fractions of a second. The actual exposure time 

 required for astronomical photography depends to a large extent upon the telescope 

 used. For further data, reference should be made to the chapter on Astronomic 

 Photography. 



