The Cine Technician (1935-1937)

48 The Joiir)uil of the Association of CineTechnicians November, 1935 Films, Filters and Filter Factors I. D. WRATTEN (of Technical Service Kodak Ltd.) One of the many difficulties that a cameraman has to overcome is that exterior sliots usually display a less marked scale of contrast than the subjects with which he is accustomed to deal in the studio. At the same time the sky is generally of much greater intensity than any other portion of the graduation scale, and it follows that in order to obtain detail in the shadows it is often necessary to over-expose the sky. This over-exposure, which tends to destroy in the picture the finer differences in intensity perceived by the eye, can be eliminated by the use of yellow colour filters absorbing strongly the blue and ultra violet rays in which the sky light is rich. This, of course, has the effect of lengthening the scale of intensities which the film is capable of rendering, whilst at the same time the action of the filter in accentuating colour contrasts further assists in differentiating white clouds against the blue background of sky. It is important to note that when filters are used the exposure must be increased, and it is necessary, therefore, to know the "filter factor" of the particular filter it is desired to use. The filter factor is the factor by which exposure with a filter must be increased over the normal exposure using no filter on the same scene and with the same film. For instance, if a filter has a factor of 4, then to use it and obtain a properly balanced exposure it is necessary to increase the exposure normally given without a filter by that factor. If, for instance, it is found that the normal exposure required is lens stop f8, then when using a filter having a filter factor of 4 it is necessary to open the lens to f4, since the lens opening is then doubled and the exposure, which varies as the square of the opening, is increased four times. In Fig. 1 is set out the exposure table for various contrast filters when using Eastman Super X, and the same type of exposure table when using Eastman Super Sensitive Panchromatic or Eastman Background Panchromatic is given in Fig. 2. As can be seen, these tables shew the filters across the top, the filter factors across the bottom, while the left-hand column shews a series of arbitrarily chosen lens stops under the heading "No Filter." All values are expressed in terms of f values. The use of these tables is extremely simple. Suppose for instance, a scene is to be photographed, both filtered and unfiltered, and that the correct exposure is found to be f5-6. Suppose further, that the Areo 2 filter is selected for use. Select in the left-hand column under the heading "No Filter" the value f5-6, project across the table in the line shewing this value until the column headed Area 2 filter is reached. At this point the value f4-5 is found. Therefore, tne scene shot unfiltered at f5-6 can be shot with the Areo 2 filter at f4-5 and equally exposed negatives will be obtained, although the filtered scene will shew a different relation between sky and foreground due to the selective absorption of the filter. It would, perhaps, be of assistance to the cameraman if definite filters could be quoted as the ones to use in certain definite instances. Unfortunately, this cannot be done, because conditions under wliicli filter shots are made vary tremendously. However, tlie guiding principle tliat governs all these cases is that to subdue a colour a filter whose colour is the complement of that to be subdued must be used, while to lighten a colour, a filter of the same colour must be used, since this filter will fully transmit the desired colour and subdue the others. Most of the filters listed in Figs. 1 and 2 are well known to cameramen and need no introduction. The 3-N-5 and 5-N-5 filters, however, require explanation. It is well known that certain exterior scenes require the use of a high aperture lens having a small depth of focus in order to obtain a particular image quality. The use of Super X and Super Sensitive Panchromatic film requires that less exposure be given than was necessary with the older types of Panchromatic materials. Now, since it is desired that a high aperture lens be used, it is obvious that to reduce exposure a reduction must be made of the angular aperture of the shutter (which is undesirable), or a neutral density must be used in front of the lens. The 3-N-5 and 5-X-5 filters were introduced to provide for cases like this. They are combinations of a yellow filter with a neutral density. FIGURE 1. Eastman Super X Panchromatic Filter Exposure Table for Daylight Expressed in F' Values. No Filter. Aero 1. Aero 2. 3-N-5. 5-N-5. G. 23A A (25) 72 2-3 2-0 1-8 2-8 2-5 2-3 1-4 l(-i 1-4 Night 3-2 2-8 2'« I-(S 1-4 1-8 1-6 1-4 Effect 3-5 31 2-8 1-8 1-6 2-0 1-8 1-6 Filter. 4-() 3-(-i 3-2 2-n 1-8 2-3 2-0 1-8 4-5 40 3-7 2-3 20 2-(S 2-3 2-0 Use at 5-(-i 5-0 4-5 2-8 2-5 3-2 2-8 2-5 f2-3 8-3 5-6 5-2 3-2 2-8 3(-) 3-2 2-8 to 8-0 71 6-5 4-0 3-(-; 4-(S 4-0 3-6 f3-5 110 9-8 9-0 5-6 4-9 6-3 5-6 4-9 12-5 11-2 10-2 6-3 5-6 7-2 6-3 5-6 16 0 14-3 13 0 8-0 7-2 9-2 8-0 7-2 22 0 19-7 18-0 11-0 lO-O 12-5 11-0 10-0 Factor 1 1-25 1-5 4 5 3 4 5 FIGURE 2. Eastman Super Sensitive Panchromatic Filter Exposure Table for Daylight Expressed in ' F' Values. No Filter. Aero 1. Aero 2. 3-N-5. 5-N-5. (;. 23 A. A (25) 72 2-3 20 1-8 Night 2-8 2-5 2-3 1-4 Mi 1(S 1-4 Effect 3-2 2-8 2 (S 1-H 1-4 1-8 1-8 1-6 Filter 3-5 31 2-8 1-8 1« 2-0 2-0 1-8 4-0 3-(-i 3-2 2 0 1-8 2-3 2-3 20 Use at 4-5 4-0 3-7 2-3 20 2fi 2-fi 2-3 f.2-3 5-6 5-0 4-.S 2-8 2-5 3-2 3-2 2-8 to 6-3 ,S(i ,S-2 3-2 2-8 3fi 3-« 3-2 f3-5 8-0 71 (v5 4-0 3-6 4-fi 4H 4 0 11-0 9-8 9-0 5-6 4-9 6-3 6 -3 5-6 12s 11-2 10-2 (v3 5-6 7-2 7-2 fi-3 KSO 14-3 13() 8-0 7-2 9-2 9-2 8-0 22-0 19-7 18() 11 -0 lO-O 12-5 12 5 no Factor 1 1 -2.5 1-5 4 r> 3 3 4