American cinematographer (Aug 1933)

132 American Cinematographer Q August 1933 Fig. 4 Editor’s Note: This is the second in a series of articles by Emery Huse, A.S.C., on Sensitometric Control. In the Sep- tember issue Mr. Huse will discuss actual motion picture laboratory negative and sensitometric control. O NE very important item which should be well un- derstood in the practical application of sensitom- etry to the control of the development of motion picture negative film is the fact that a sensitometric strip which has been made under precisely controlled conditions of time and intensity can only give data as to the degree of development obtained. Thus, such a sensitometric strip developed with an exposed picture negative will show a definite gamma value for the strip, but will not give any precise information as to the contrast of the picture nega- tive developed along with it. Contrast in the negative is not only a function of gamma but also of the lighting bal- ance in the set at the time of exposure. If at all times the brightness balance would be maintained constant in the various scenes to be photographed, then the sensitometric strip would give a true indication of the contrast of those scenes after development. However, this is not possible. Each and every scene of different subjects has an inherent brightness contrast characteristic of its own and upon con- trolled development the resultant densities in each scene indicate the contrasts which were in those scenes. The point which is being stressed is the fact that because pic- ture negatives are developed in a solution which gives a definite value of gamma from the sensitometric control strip, it does not follow that these negatives have the same degree of contrast as shown by the gamma value of that strip. They bear a very definite relationship to each other and under certain conditions can be used synonymously, but this condition does not always hold. If we refer to Figure 4 it will be readily observed that the straight line portion of the H and D curve from which gamma is determined exists only between points B and C. This straight line por- tion contains only part of the various densities which go to make up the complete curve. This being the case, the densities which are beyond either extreme of the straight line must play some part in the photographic rendering of subject or scene. When one looks at a motion picture on the screen and studies it for contrast, all thoughts of gamma disappear. What the observer is looking at is the relation- ship existing between the highlights and shadows. Sensi- tometrically this refers to the toe and shoulder portions of the H and D curve. Therefore, it seems evident that in studying contrast sensitometrically that densities which lie on the toe and shoulder must be considered. In evaluating Sensitometric contrast it is necessary to determine the density at some fixed point on the toe and shoulder and contrast can then be expressed in terms of a density value. The difference between the density chosen on the toe and that on the shoulder gives this data. It has been sensitometric practice to consider the extremes of the curve as the density at those points on the toe and shoulder where the slope is equal to .20. From the curve as shown in Figure 4 a .20 gradient on the toe and shoulder would be slightly above point A and slightly below point D. The word “gradient” signifies the slope of the curve at any given point. How- ever, along the straight line of the curve the gradient is constant and is equal to gamma. As was previously indicated, there is one instance in prac- tical sensitometry where gamma and contrast can be used synonymously and that is in the sensitometric control of sound track. Inasmuch as the exposures on the track are based upon the straight line portion of the H and D curve, and for most recordings densities in the actual track fall within the limits of the straight line, then in this instance where the maximum and minimum densities recorded are completely included in the straight line, it is quite simple to see that contrast and gamma are identical. Every major studio or commercial laboratory in Holly- wood is adequately equipped with or has access to the in- struments which have been described in the early part of this paper. Furthermore, each laboratory has a man, or several men, taking care of the sensitometric routine. It can be stated rather strongly that with the advent of sensi- tometric control motion picture film processing has attained a degree of perfection which has not been hitherto pos- sible. Of course all of the current quality should not be laid to sensitometric control because during the past sev- eral years much has been accomplished from the standpoint of improved machine development, photographic emulsions, developers, and processes which have aided in this improve- ment. However, it is the candid opinion of the author that sensitometric control has revealed deficiencies in the process- ing systems which have not heretofore been observed, or if observed, were not properly taken care of because there was no technical control available to indicate the direction in which improvement should be made. In attempting to convey clearly a concise picture of the actual control methods in use it is felt that the subjects of control for negative, positive and sound track should be treated individually. Furthermore, that this paper might contain more than the opinion of the author, data will be presented from actual production laboratories, which data will show clearly the degree of consideration which is given to sensitometric control. Before discussing actual laboratory data there is an im- portant consideration which should be given to the develop- ment of negative film. In Hollywood there are two dis- tinctly different methods by which negative film is devel- oped. One method is that of a constant time of develop- ment. The other method is that which is colloquially termed the test method. By the constant time method is meant that the developing solution is maintained at a definite control gamma, as shown sensitometrically, and