Journal of the Society of Motion Picture Engineers (1930-1949)

Jan., 1930] PROBLEMS OF WIDE FILM PICTURES 57 Some additional illumination, however, is possible by using an astigmatic condenser, one whose focal length in one meridian is shorter than its focal length in the other principal meridian. Such a condenser can be realized by employing one cylindrical surface, as we have done for several years in one of our ophthalmic instruments, or by employing a toric surface. Such a condenser will yield a spot of light in the plane of the film such as is shown in Fig. 5. A preliminary investigation subject to possible correction indicates a gain of something like 25 per cent obtainable in this manner. If, now, the arc be run at something like 150 amperes with condensers as described above a satisfactory illumination will be found possible. It still remains a question as to just what degree of illumination will be required. It is possible that the relatively enormous FIG. 5. Elliptical spot of light results in less loss by interception than in the case of a round spot. picture on the screen may prove more satisfactory at a level of brightness lower than we have been accustomed to in the smaller picture. Certainly, a projected picture of, say, 23 X 46 feet illuminated as brightly as some of the' news reels we see might be expected to raise the general illumination of the theater to an undesirable level. For the projection of the pictures ordinary projection lenses are entirely out of question except in the longest focal lengths because of objectionable curvature of field. It happened that I had been working on an improved form of lens for the shorter focal lengths for the projection of ordinary film when the demand came for lenses to project the large pictures. The design had progressed to the point where it was possible to offer lenses of 4 in. equivalent focal length