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

added a corrector plate. Parabolic mirrors with corrector plates have been used. Only recently (Bouwers & Maksutov2-3), however, has the spherical aberration of the spherical mirror been corrected by the addition of commercially reproducible all-spherical correctors, thus making available the very real advantages of reflective optics on a wider scale than ever before. Perhaps the most important and most obvious advantage of the spherical mirror is the complete absence of chromatic aberration. If a diaphragm is placed at the center of curvature of a spherical mirror no coma, astigmatism or distortion can occur, for the straight line containing the center of curvature parallel to any given ray of light may be regarded as an axis of the system. There is curvature of field convex to the incident rays, the radius of field curvature being about equal to the focal length for objects at infinity. Field curvature is rarely a problem when the field of view is restricted. Where field coverage relative to focal length is large, suitable field flatteners or curved film may be used. An inconvenience of the spherical mirror is the reversal in direction of the light rays. There is some loss of light since the object or image receiver intercepts part of the useful beam. The "shadow effect" can be serious with an extensive field of view and small relative aperture. The reversal in path, however, is a real and distinct advantage for long focal length objectives because the "folding" reduces total length to about one-third that of a refractive optical system of equal focal length, thereby giving a more compact and lighter system. Reflective optical systems require different mounting techniques and are much more sensitive to misalignment and spacing changes than refractive optical systems. The correction of spherical aberration is achieved by the addition of a weak spherical corrector lens closely concentric with the primary spherical mirror. With spheri cal aberration corrected, with coma, astigmatism and distortion eliminated by choice of stop position, and with no chromatic aberrations present, the resolution is primarily limited by diffraction. The spherical mirrors are mounted in a manner to eliminate focus shift caused by temperature variations. Such mounting is imperative by reason of the very sensitive air space between mirror surfaces. Wide-Angle Lenses The 84° and 142° wide-angle lenses for 16mm film and 35mm film listed in Tables IV and V have proven to be invaluable where an extreme field of view is required. The systems of the lenses are basically reversed telephotos in form. An extreme depth of field is possible for the lenses even at the maximum //1. 5 and //2.0 apertures because of the very short focal lengths and small residual aberrations with the exception of distortion. The resultant distortion is characteristic of extreme wide-angle lenses of high relative aperture. Successful application of the 142° lenses in moderately high-speed motion picture photography is discussed by Bauer and Blake of the Douglas Aircraft Go.15 The lenses are used to record automatically instantaneous instrument data shown on a wide variety of instruments over a large area where the distance from panel to camera is restricted. In many cases the instrument or objects must be located in azimuth and elevation within the distorted field, necessitating correction charts to compensate for the distortion present in the optical system. The charts are made by photographing an accurately drawn rectangular grid pattern at a predetermined reduction with the lens whose field is to be plotted. The grid spacings are drawn, in both azimuth and elevation, to subtend a given angle in the field of the lens. The photograph obtained is then enlarged with a distortion-corrected lens to a specified 500 December 1952 Journal of the SMPTE Vol. 59