300 HANDBOOK OF PHOTOGRAPHY 



exposure factor also depends upon the light source and the film. For this reason two 

 filter exposure factors are usually given for the most common light sources, one for 

 daylight and one for tungsten illumination. 



Having worked out a graphical explanation for filter exposure factors, we now 

 propose to reinterpret the preceding section in less technical terms more familiar to the 

 average photographer. 



When a light filter is used in combination with a given light source and photo- 

 graphic film, the time of exposure must be increased if the film is sensitized for rays 

 corresponding to those colors which the filter absorbs. (In this, as well as the pre- 

 ceding discussion, no account is taken of loss of light by reflection from the surfaces of 

 the filter or of absorption by the glass plates, cement, etc., except as these factors enter 

 into the over-all transmission characteristic of the completed filter.) This increase 

 in exposure is most necessary when the colors (wavelengths) absorbed by the filter 

 are those for which the photosensitive material is most sensitive. The magnitude of 

 the increase in the exposure which is necessary when a filter is used, may be said to 

 depend upon the transmission characteristics of the filter. In a broader and less 

 accurate manner, it may also be said that the filter exposure factor depends upon the 

 color of a filter, for the color is entirely dependent (in the visible spectrum, of course) 

 upon the absorption of the filter. 



Items Affecting Filter Expostire Factor. — The increase in exposure required when 

 a filter is used depends upon the density of color of the filter, since this determines the 

 amount of absorption for a given color (wavelength). Thus, for example, although 

 all yellow filters absorb blue light, a dense- or deep-yellow filter absorbs more blue 

 than a light-yellow filter if both filters have spectral-transmission characteristics of the 

 same shape but different magnitude. Consequently the deep-yellow filter would 

 have a greater filter exposure factor than the pale-yellow filter. (This effect is not 

 shown on the set of curves, but might easily be indicated by means of another fiilter- 

 transmission curve having less transmission at all wavelengths than the G filter — 

 representing a deeper or darker filter). 



Another factor influencing the exposure through a given filter is the sensitiveness 

 of a film for particular colors. If, for example, a film is relatively sensitive to all 

 visible colors, as panchromatic films are, the filter exposure factor for a yellow filter 

 which absorbs only the blue will be much less than if the film were of the ordinary 

 (noncolor-sensitive or orthonon) variety and sensitive almost entirely to the blue rays. 

 Thus, when used with a daylight source of light, the Wratten No. 8 or K2 filter has an 

 exposure factor of 12 for ordinary or orthonon materials but a factor of 2.5 for ortho- 

 chromatic and of only 2 for fully panchromatic materials. 



A practical consideration in photography is the time of day in which outdoor 

 scenes are taken. As daylight contains more blue-violet at noon than in the morning 

 or later afternoon, the increase of the exposure time will be greater at noon than in the 

 morning or the afternoon for a yellow filter. This statement refers only to the 

 increase in exposure due to the filter factor; in practice the luminous intensity at noon 

 is much greater than in the morning or afternoon, so that the absolute exposure is 

 likely to be less than in the morning or afternoon. Similar fluctuations in spectral 

 (color) distribution are caused by the seasons, as well as by the time of day, and by 

 atmospheric conditions also. In general, where yellow filters are used, a good prin- 

 ciple to follow is to increase the filter exposure factor the more blue the light source is 

 and the less the film is sensitive to colors other than blue. 



It is, therefore, impossible to state the correct increase of exposure time for every 

 light condition or even for every filter, and when attempts are made to indicate filter 

 factors, it should be understood that these factors are approximate and may require 

 some deviations, depending upon the characteristics of the light source and film. 



