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Because gamma is the most important factor in a tone-reproduction system, it is proper to consider all other characteristics at a given gamma value. Fortunately, in the motion picture field the gamma values used for negatives and prints are fairly narrowly prescribed. The overall gamma from subject to print should be approximately 1.3, that is the product of negative and positive gammas should have this value. The negative gamma is usually 0.65 ; it varies in practice usually from 0.6 to 0.7. The print gamma is usually just over 2.0 in the case of type 1301 film. The two sensitive materials used in this work give characteristic curves with well defined straight line portions and the gammas may be readily ascertained.
It is of great importance, especially when making the original negative, that an image with the correct gamma and density should be obtained with a minimum of exposure. The sensitivity of a photographic material is determined both by the properties of the emulsion and by those of the developer used, but we are concerned here only with the effects due to the developer. No account is necessary here of the pitfalls that beset the determination of emulsion speed. In the present work, the method of inertia speed was selected. Since a fixed gamma was to be used, the inertia speed method would indicate the lateral displacement of the characteristic curve with respect to the log E axis.
Toe Shape
The question of sensitivity is also bound up with the shape of the toe of the characteristic curve. The shape of the toe of the positive material is also of importance in that it affects the reproduction of fine tones in the highlights of the picture. A reproduction of an original subject of wide brightness range appears harsh wThen seen on the screen if the photographic characteristics are nearly linear. In fact, the highlights need a reproduction of lower contrast than the medium and dark tones. Any method of quantitative assessment of the shape of the characteristic curve must be arbitrary in some degree. In the present work the shape of the toe is expressed numerically as the log exposure separation of the curve above the inertia point from the extended straight line. This is illustrated in Fig. 2, where C is the inertia point and AB is the log E separation of the curve from the straight line. This quantity AB will be referred to as the Toe Displacement.
Two important properties of a developer remain to be considered. These are its rapidity of action in producing the desired gamma and the fog level that it creates.
Mode of Procedure
Sensitometric strips exposed on the II B sensitometer were developed for increasing times in any one developer so as to obtain a family of characteristic curves. Development was carried out at 20° C. under conditions of vigorous agitation. When the processed strips had been measured, curves could be derived showing the relationship between gamma and inertia speed, toe displacement, fog, and time of development, etc.
V. RESULTS OF EXPERIMENTS
The results obtained according to these criteria can be seen in the following figures.
Inertia Speed for Negative Film
Fig. 3 shows the inertia speed for the negative film at a gamma of 0.65 as a function of Klon and hydroquinone proportion. The inertia speed