Journal of the Society of Motion Picture and Television Engineers (1950-1954)

The concept of the speed of a photographic material has inescapable psychophysical connotations. One material is usually regarded as having a higher speed than another when it will produce a satisfactory photographic image with less exposure. In order to place the concept of speed on a workable basis for black-and-white motion-picture negative films, however, a more explicit statement is required. Such a statement is the following, which is analogous to the statement regarding the concept of speed in the "American Standard Method for Determining Photographic Speed and Exposure Index," Z38.2.11947, applying to roll and sheet films: The speed of a motion-picture negative film for continuous-tone black-and-white pictorial photography is inversely proportional to the minimum exposure required to produce a negative from which a positive print can be obtained that will give a projected screen image of high quality. This definition represents a logical basis for a sensitometric speed criterion for motion-picture negative films. The necessary first step toward the realization of such a sensitometric criterion is a psychophysical study of the manner in which the quality of the projected positive screen image varies with the camera exposure given the negative. A study has been completed using photographic reproductions of sixteen outdoor scenes and eight studio scenes. An exposure series was made of each scene on each of two black-and-white motion-picture negative materials. Exposure series of most of the outdoor scenes were also made on each of two reversal color films. The tone-reproduction quality of the final positive images obtained with each material was judged by about thirty observers. Averages of all of the judgments were obtained and used in plotting curves showing the tone-reproduction quality of the positive image as a function of the logarithm of the camera exposure. Choice of Scenes The scenes which were photographed were planned and constructed with two principal aims in mind. As far as possible, it was desired to conform to lighting practices followed in professional motion-picture work. It was also desired to have as much variety as possible in the types of scenes which were used. It was not difficult to meet both of these requirements for the studio scenes, but the outdoor scenes presented some problems in lighting which were caused by changes in the position of the sun during the time required to make all of the necessary exposures, meter readings and scene luminance measurements. The fill-in lighting required to reduce the ratio of highlight to shadow illumination on the models in the outdoor scenes was obtained by using reflectors. The lighting problem was somewhat simplified by the fact that the people in each scene remained motionless. Experience has shown that it is easier to evaluate the relative tone-reproduction quality of each scene in an exposure series if the people in the scene remain in one position throughout the series. The hope has sometimes been expressed that an "average" scene could be set up which would be so nearly typical of the scene types encountered in motion-picture work that it could be used in making direct camera exposure tests, making unnecessary the considerable expenditure of time and effort required to carry out experiments involving many different scenes. There are several reasons why this is not feasible. The variables which change from scene to scene, and the combinations and interactions of these variables, are so numerous that it is practically impossible with any reasonable degree of certainty to isolate the significant variables for consideration. For example, although it is quite probable that the exposure required to produce optimum quality will depend upon the lighting ratio, the nature of this Sorem: Effect of Exposure on Tone-Reproduction 25