MOTION-PICTURE PHOTOGRAPHY 



695 



given filter is the fiilter factor. It is the inverse of the percentage of light transmitted 

 by the filter; thus if 80 per cent of the incident light is absorbed and 20 per cent trans- 

 mitted, the filter factor equals 5.0. This 

 multiplier, however, is not to be regarded as 

 a constant of the filter, for obviously it de- 

 pends also on the spectral sensitivity of the 

 film and the spectral composition of the 

 light. Specifying a filter factor is meaning- 

 less unless these two elements are defined. 

 For example, the No. 3 filter described would 

 have a much higher filter factor with a blue- 

 sensitive emulsion than with a panchromatic 

 emulsion, the sensitivity of which extends 

 into the region where the filter has no 

 absorption. Likewise for light in which 

 blue-violet predominates the filter factor 

 would be higher than for tungsten illumi- 

 nation rich in yellow, orange, and red. This 

 particular filter has a low multiplying factor 

 (1.25) for light of normal daylight quality 

 and a panchromatic material, i.e., it may be 

 used under these conditions practically with- 

 out increase in exposure. 



Figure 41 shows the effect under the 

 same conditions of four widely used filters, 

 in the following order from top to bottom: 

 Nos. 5, 15(G), 23A, and 72. The absorption 

 increases in the same order. For purposes 

 of comparison these four filters and the No. 

 3 are listed in Table III. -pm. 41.— Effect of Wratten filters on 



Filter factors for three panchromatic spectral response of Eastman panchro- 

 negatives furnished by the Agfa Ansco Cor- matic film with daylight source of illumi- 

 poration are given in Table IV, which has ^ft^^^- i- ^ flter; 2, 15(G) filter; 3, 23A 



., f , , ,. PI filter; 4, 72 filter, 



been compiled by the manufacturer of these 



films. 



In general, a color filter which transmits light of a given color will lighten that color 

 on the print, while a filter which absorbs light of a given color will darken that color. 

 Consequently the following effects are typical: 



Red filters tend to lighten reds markedly, also to lighten orange and yellow, to 

 darken blues strongly, and to leave green unchanged. 



Yellow filters tend to lighten yellows most, to lighten orange and green to a lesser 

 degree, to darken blues, and to leave reds unchanged. 



Green filters tend to lighten greens and yellows, to darken red, and to leave blue 

 and orange unchanged. 



The filter factor and the increased exposure required necessarily increase with depth 

 of color, since the darker the filter the more light is lost. 



Among the applications of filters itemized at the beginning of this section, the 

 elimination of haze sometimes assumes importance, particularly in aerial cinematog- 

 raphy. Haze and mist are caused by suspended water and dust particles scattering 

 the violet and ultraviolet rays of the sun. The film being sensitive to these short 

 wavelengths, a veiling of the distant portions of the landscape results, unless measures 

 are taken to prevent the fogging light from reaching the film. Strong yellow filters 



