The international photographer (Jan-Dec 1931)

Six The INTERNATIONAL PHOTOGRAPHER March, 1931 portion of red light than does daylight, and the difference in speed of the two films indicates that the super sensitive film must possess greater sensitivity to red light than the present type of film. It is mainly for this reason that the tungsten speed of the super sensitive when compared to the present type of film is greater than for a similar comparison to daylight. The difference in color sensitivity of the two types of films is shown in Figure 2. This figure shows prints of spectrograms of the two types of film when exposed to tungsten. Speed comparisons should not be drawn from these prints as the prints are so made to show the regions of the spectrum to which each emulsion is sensitive. The figures given represent wave lengths, and beginning at 40 in the blue violet region we have increasing wave lengths through the blue violet, blue, green, yellow, and orange as far as the deep red given at a wave length of 68 microns. The super sensitive film shows an increased concentration of sensitivity in the region around 64 microns. The super sensitive film confines its sensitivity to the definitely visible portion of the red end of the spectrum, while the present type of films shows an extension into the deep red and encroaches upon the near infra-red region. This concentration of visible red sensitivity gives a marked advantage to the super sensitive as it is the extension of red sensitivity into the region of longer wave lengths which is a contributory factor in the production of chalky highlights under tungsten illumination. This is eliminated to a marked degree with the super sensitive film. A complete study of an emulsion's sensitivity to color necessitates actual speed measurements to the three major portions of the visible spectrum, namely to blue, to green, and to red light. For the purpose of obtaining such information actual speed tests, similar to those shown in Figure 1 and later verified by practical exposure, were made to daylight through the No. 49 (blue), 58 (green), and 25 (red) filters. Speed values determined from such tests show that the super sensitive film has 75 per cent greater speed to the blue, 200 per cent greater for the green, and from 400 to 500 per cent greater for the red exposures. Such sensitivity naturally lends to a better and more intelligent use of filters, either for straight photography or for trick work. Increased Speed Another important consideration in the comparison of present and super sensitive film pertains to contrast and the rendering of shadow detail and softer highlights. Figure 3 shows for tungsten exposures the difference in time of development between the two types of film to produce equal degrees of contrast (gamma). These curves are of equal gamma and the data contained shows that it was necessary to develop the super sensitive film three-quarters of a minute longer to produce this effect. Furthermore, greater density is picked up in the low exposure region. This is mostly accounted for b,r the increased speed of the super sensitive emulsion, but it is this ability to pick up and differentiate between these low intensities which gives the high order of shadow detail rendering which is shown by this super sensitive emulsion. On the other hand, in the region of high exposures it will be observed that the super sensitive film shows a tendency to break into a shoulder, while the present film continues as a straight line. This is at least true for the series of exposures shown in the figure. This break into a shoulder lends to softer highlight rendering and still permits of very definitely separating highlight intensities and thus produces details in this region. Figure 4 shows in much more detail the relationship existing between contrast (gamma) and time of development. These curves, made from exposures to tungsten, represent what are commonly referred to as time-gamma curves and they show the rate at which gamma builds up with increasing time of development. The rate of increase of contrast with increased development time is appreciably less for the supersensitive film, as will be shown by a study of the curves in Figure 4. This means that in the handling of Tungsten Hi ■TOwjiillhli 1 , 44 52 56 ;,i.iii,H;k! i,. 60 64 1 8)*l IllllH Present Films ^mmmm^^^mm m&mm^^+u* . . .-■■.A.. | «atIC&677 V MSi'iilii'ldim iiiiiL 52 56 60 1 64 (1 IS Exposure Tungsten It 12 1 0 08 > l z Development: i sMin. in Borax 2 S»A( K^\-^^ OS ft i Tresent Films z Supersensitive Film 02 "Relative Log E 0.0 OS 0.0 0.9 12 IS l» 2.1 Figure J the film during development there is relatively little chance of either under or over developing. Errors of the order of 25 per cent in development time will have a much less marked effect on the super sensitive film. In other words the super sensitive film gives to the laboratory man that one thing which is so important to him and which is colloquially referred to as "development latitude." There is just one caution which should be mentioned at this time. Due i "Present Films 2 Super Sensitive Film a a It 15 18 zi Minutes in Boyax Super Sensitive Film Figure 2 figure U to the increased sensitivity of this emulsion, the handling of this film cannot be successfully accomplished unless the illumination from the present safelights is reduced appreciably. The ideal condition under which to handle this film would be total darkness, and no doubt this condition will prevail inasmuch as many camera loading rooms and laboratories which process negative on machines now operate in almost, if not total, darkness. It is felt, therefore, that this will not work any great hardship on the laboratory. However, this word of caution is considered necessary because of the greater increased speed both to white and to colored light of the super sensitive emulsion. What Super Sensitive Does It is felt that a summary of the outstanding features of the super sensitive film will bear repeating. 1 — Super sensitive film is twice as fast to daylight and three times as fast to tungsten light as the present type panchromatic films. 2— Super sensitive film shows 75 per cent more speed to blue light, 200 per cent more to green light, and from 400 to 500 per cent more to red light. 3 — Super sensitive film exhibits an appreciably softer characteristic than present films. For the same time of development the super sensitive film gives lower gammas. This makes it advisable to develop the super sensitive film longer if the same degree of contrast as now accepted is still desired. 4 — Super sensitive film must be handled at a much reduced light intensity in the dark rooms.