International projectionist (Jan-Dec 1954)

The Optics of CrnemaScope A. By RICHARD ALTMAN Scientific Bureau Bausch & Lomb Optical Company Although many general articles on CinemaScope have appeared, relatively few have concerned themselves with the optical principles involved. The more familiar type of lenses such as those used in projection of motion pictures enlarge the image on the film and transfer it to the screen. The picture on the screen is the same as that on the film (Fig. 1) only the size has been changed due to the magnification of the projection lens. The magnification is the number of images on the film needed to cross over the image, and will be the same in all directions if the picture on the film is to be reproduced faithfully on the screen. In CinemaScope the picture on the screen is different from that on the film (Fig. 2). Circles on the film are projected oval, showing a difference in magnification between the horizontal and vertical directions. The lens which accomplishes this is called an "anamorphic" because it "reshapes" the image. Alteration of Image by CinemaScope Lens In the illustrations the lens is shown projecting at three times magnification. When a CinemaScope lens is added the magnification remains at three times in the vertical direction but is now six times in the horizontal. The focal length of the projection lens is thus effectively halved in the horizontal direction, while remaining unchanged in the vertical direction. The complete optical system for CinemaScope includes a conventional camera lens plus a CinemaScope attachment for recording a squeezed image on the film, and a conventional projection lens plus a CinemaScope projection attachment to expand the image to normal proportions on the screen. Since all CinemaScope pictures are photographed in a squeeze ratio of two times, they must be projected by an anamorphic attachment fixed at two times expansion so that the final picture will not be distorted. CinemaScope attachments must have a high degree of optical correction in order that the quality of the projected image will not suffer. A cylinder lens system was chosen by Bausch & Lomb over a variable-type prism system as affording the best quality imagery at any given squeeze ratio. In addition, the projection attachment must work well with lenses of various focal lengths. For instance, the anamorphic attachments must have sufficient diameter to maintain the optical speed of long FIG. 3. CINEMASCOPE OPTICAL SYSTEM -f OBJECT -^ ^ CAMERA ^ .^n SCREEN PROJECTOR focal-length lenses and yet allow enough light to get to the corners of the screen when used with short-focallength lenses. A single attachment, meeting these mechanical specifications would require a very bulky lens. CS'-cp'^ 'ens No* a Correcfive Unit For this reason Bausch & Lomb supplies two forms of CinemaScope projection attachments (Fig. 4). Projection attachment I for lenses of 3 to 5-inch focal length in 2.781-inch barrels, and attachment II for lenses of 5 to 7-inch focal length in 4-inch diameter barrels. The projectionist is thus assured of top quality projection with a minimum of extra bulk to handle. Furthermore, the attachment will not correct any defects that already exist in his projection lens. In fact, those defects are more noticeable when the CinemaScope attachment is added because of the extra magnification introduced in the horizontal direction. For this reason, lenses that may appear to give an acceptable image in normal projection may give poor imagery in Cinema FIG. 1. MAGNIFICATION OF FAMILIAR TYPE LENS DIRECTION Three times magnification in botli vertical and horizontal directions. FIG. 2. CINEMASCOPE MAGNIFICATION w- FILM LENS ^ ,-n lb* CINEMASCOPE ATTACHMENT SCREEN 3 TIMES 6 TIMES The CinemaScope attachment raises the horizontal magnification two times 10 INTERNATIONAL PROJECTIONIST • AUGUST 1954