International projectionist (Jan-Dec 1949)

The 35-mm Projection Positive Film By ROBERT A. MITCHELL /. Types of Film The first of a series of articles relating to the anatomy, characteristics, and care of various film stocks as used in both studios and theatres. MOTION picture film differs basically from ordinary snapshot film only in its physical dimensions and requisite marginal perforations. Each is a photographic film consisting of a plastic base coated with sensitized emulsion. The light-sensitive coatings of both cine and still film are essentially the same: one type of plastic base may be used for both kinds of film; and each is exposed, developed, and "printed out" on other sensitized photographic materials by processes which, as far as fundamental principles go, are analogous with each other. The picture which is brought out by developing a correctly exposed camera film (cine or still) has a characteristically unnatural appearance. All values of light and shade are reversed: the dark objects in the original scene come out light on the film, and the highlights come out dark. The tonal values of the camerafilm picture are therefore negative in relation to the tonal values of the original scene. Negative pictures are obviously illsuited for exhibition purposes, either in snapshot albums or on motion picture screens. So, in order to get faithful representations of the original scene (images in which the highlights and shadows correspond with those of the photographed scene) the negatives are printed on other pheto-sensitized materials which are subsequently developed in about the same way that the original negatives were developed. The 'Reversal' Process With the preparation of the print, two "reversals" have taken place. The tonal values of the printed-out picture are negative to those of the camera-film picture, but positive to those of the original scene. The "positive" made by printing the "negative" accordingly provides a reasonable facsimile of the scene viewed by the camera's eye. A snapshot negative is ordinarily printed on paper coated with photographic emulsion ; but a motion picture negative must be printed on emulsioncoated perforated film. There is no great difference between negative and positive "raw stock," the term applied to undeveloped motion picture film. In fact, it would be possible to photograph a motion picture on positive stock and print it on negative stock, but the quality of the picture might be very poor in such a case. There are many different grades of emulsion available, with each having been produced to give the best results when used in the application for which it is specifically intended. Types of 35-mm Cine Film Because several types of plastics are used for the emulsion-supporting base of film, we sometimes find it convenient to classify motion picture film according to the nature of the base. The two principal categories of cine-film base are "inflammable" and "safety." A number of specific types are found in each of these two classes: (A) Inflammable (Nitrate) Base: Cellulose pentanitrate (nitrogen content 12.8%) ; Cellulose tetranitrate (nitrogen content 11.1%) ; Cellulose trinitrate (nitrogen content 9.1%). [Ordinary nitrate film base is a mixture of these three nitrates, the tetranitrate predominating.] (B) Safety Acetate Base: Cellulose diacetate, Cellulose acetopropionate, Cellulose triacetate, Butyril acetate, . . . and others. This system of classification is extremely useful to all who process, repair, project, or store motion picture films. Solubility in cementing fluids, flammability, tensile strength, flexibility, resistance to the action of heat, permanence, etc., are among those properties of film which are either wholly or largely determined by the base.1 But those who are concerned only with the photographic aspects of motion pictures consider film from an entirely different point of view. The cameraman, for instance, is concerned primarily with the light-recording characteristics of the emulsion, and he can afford to ignore completely many of the properties of film which are of vital interest to the projectionist. The processing technician, however, makes use of both points of view, the physical and the photographic. The film laboratory may therefore be regarded as a bridge directly connecting the complementary fields of cinematography and projection. Film is classified photographically as "positive" and "negative," a distinction based not on any fundamental differences between the two kinds but rather on the applications for which they are manufactured. In general, a film made for use in cameras is negative stock, while that intended for use in printers (and, later, projectors) is positive stock. But positive stock is used as a negative in soundtrack recorders and in title cameras, and negative stock is used as a positive after it has been developed by a special reversal process. Now, both positive and negative stock include many different grades and types of emulsion, permitting further classification : (A) 35-mm Positive Stock: Regular positive, Sound positive, Duplitized positive, . . . and others. (B) 35-mm Negative Stock: Orthochromatic negative, Panchromatic negative, Extrasensitive panchromatic negative, Background negative, Infra-red negative, Reversal negative, Duplitized color negative, Monopack color negative, . . . and others. Positive Raw Film Stock On the whole, positive emulsions are "slower" (less sensitive to light) than negative emulsions. Regular Positive, for example, is only about 1/10 as "fast" as Panchromatic Negative. This means that ordinary positive stock requires approximately 10 times the exposure needed by Panchromatic Negative to produce the same density of image. The wide latitude (high contrast-factor) of positive stock compensates for the comparatively low range of contrasts in negative picture images. This characteristic also makes positive raw stock useful as a "negative" for photographing movie titles in which the very highest degree of contrast between the white lettering and the black background is desired. The color response of positive stock does not match the color sensitivity of human vision. Red, orange, and yellow look very bright to the eye, but positive emulsion is so little affected by these colors that when film of this type is used 1 Parts V and VI of this series are devoted to film-bise characteristics and their significance in projection practice. 3 INTERNATIONAL PROJECTIONIST • November 1949