Motion Picture News (Mar-Apr 1923)

1708 Motion Picture News Description of the Motion Picture Camera Explanation of the Basic Principles and Salient Feature of Apparatus Extracts from " Motion Picture Photography " by Carl L. Gregory, published by Xew York Institute of Photography Chapter IV. LONG before motion pictures were dreamed of, philosophers and medical men were conscious of persistency of vision. They knew from their experiences and the experiences of others, if they looked at a bright object, such as the sun or a lighted lamp and turned their eyes to a dark corner the image, or at least a bright spot, would remain before their eyes for a few moments. The brain retained the illumination that the eye had sent to it for a few moments. Experiments proved that this persistency of vision did not occur in the retina of the eye. Close inspection of the retina showed that the picture projected thereon by the lens of the eye vanished the instant the entering ray was cut off. Therefore scientists stated definitely that the illusion was centered in the brain. No further explanation has been made. No human being or animal has ever been known to be without this peculiar trait. No human being or animal has been known to lose this persistency of vision. If a mortal could be found who did not possess it, when looking at moving pictures, he would see not pictures in motion but a number of " still" or inanimate pictures following one another very rapidly, each one perfectly still for about a sixteenth of a second. What Motion Pictures Actually Are Motion pictures are simply a number of snapshots run before a strong illuminating light and projected, by means of a powerful lens, upon a white screen or surface. Each picture is arranged so that it will stop for a fraction of a second and then move on, succeeded by another slighly different in appearance. The brain retains the image of the first picture and when the image of the second is telegraphed to it, by the sense of sight, the two blend and overlap and the spectator imagines he has seen but one image. The camera in which the pictures are taken is similar to the projecting apparatus but instead of the light rays being emitted from the machine, as in the the case of the projecting machine, they are gathered in or admitted through the lens. The rays fall upon a long strip of sensitized film, the same as that used in small hand cameras, made into a continuous roll which is fed past the lens intermittently at the rate of sixteen exposures a second. A revolving shutter is used in both camera and projector to cut off the light while the film is moving and a new section is being drawn into position before the lens. The motion picture camera is similar to the ordinary camera with the exception that it is provided with a mechanism for making exposures in rapid success on a ribbon of film. Sixteen pictures per second has been adopted as the standard speed for taking and projecting motion pictures. This rate was adopted after a long series of experiments to ascertain the least number of pictures necessary to produce upon the screen a moving picture which would not offend the eye by the flicker or pulsation due to the intermittent succession of light and darkness which produces the illu sion of motion. If the number of pictures thrown upon the screen is less than sixteen per second, the persistency of vision is not sufficient to carry the impression of light over the intervening period of darkness. Although the eye may not be able to distinguish that the light is completely cut off while the next succeeding picture is being drawn into place, there is an unpleasant pulsation commonly called "flicker" which is very fatiguing and annoying. By increasing the number of alternate dark and light periods per second the persistency of vision is able to bridge the gap between the successive periods of light thrown on the screen. As the flashes increase in their rapidity, they gradually merge into a sensation of continuous light upon the screen without perceptible pulsation or flicker. Flicker Perceptible at Sixteen Per Second At sixteen pictures per second flicker is very perceptible so that many of the first cameras made were constructed to take many more than sixteen pictures per second. Some of them made as many as sixty-four exposures and used a film four times the area of the present standard. With the small returns obtainable from the exhibition of motion picture films in those days, this rendered the expense of taking motion pictures almost prohibitive. The present narrow width of film was adopted to cut down expense. It was also found that it was not necessary to take so many pictures to produce satisfactory illusion of motion. However, flicker is unpleasant when the number of light flashes is less than thirty per second. Sixteen pictures per second produce a satisfactory illusion of motion so instead of taking and projecting thirty or more pictures per second, a second blade or flicker blade was placed upon the shutter of the projection machine. This intercepted the light for an instant while the individual pictures stood still upon the screen so that there were two flashes of light for each picture. Any camera mechanism which records the successive pictures upon the sensitive film is satisfactory — there is no need of a flicker blade except to make a perfect record for reproduction. It is highly desirable that the pictures be Diagram of the mechanism of the Universal Camera. A single sprocket camera with, harmonic cam movement accurately spaced at the standardized distance of three-quarters of an inch from center to center or sixteen pictures per foot. Each successive picture when thrown upon the screen will be as nearly as possible in perfect register, that is in exactly the same place upon the screen. If this is not done an unpleasant jumpiness or wavering of the picture will result. In recording, that is in photographing, a motion picture at the rate of sixteen per second, there are several operations in making each frame or picture which must be accomplished in one-sixteenth part of a second. It is not possible to utilize all of this sixteenth part of a second in making the exposure because the film must be drawn down into position for a succeeding epxosure before the next sixteenth part of a second. During this very short period of time, it is necessary to cut off the light from the lens by means of the shutter, draw the film down accurately just threefourths of an inch, hold it in place, and expose it to the image from the lens long enough to impress that image upon the sensitive surface, then completely cover the film exposed in the frame aperture before repeating this cycle of operations. All must take place in the sixteenth part of a second. It will be appreciated that a mechanism which fulfills these conditions must be accurately and substantially constructed and be able to perform this cycle of operations many thousands of times without appreciable wear. It is possible to construct an intermittent mechanism which will draw the film down so rapidly that only a fifth or sixth part of this sixteenth of a second is used in changing the film, but such a mechanism wears out many times more rapidly than one which takes a longer time to pull the film down for the next exposure. Accepted Practice in Camera Construction In constructing a camera, therefore, it has been the generally accepted practice to use an intermittent mechanism, comparatively slow in moving the film up and to make up for its slowness by increasing the " rapiidty " or "speed" of the film. Although these words are not correct, they are often used to indicate the sensitiveness of the photographic emulsion. Sensitiveness of the film is its ability to record the lens image in a given time. There are many types of camera movement, but the best of these is probably the harmonic cam. This is often called the Lumiere, or the Lumiere-Carpentier movement, as it was first used in a camera of that name. The harmonic cam is a triangular cam with curved sides, working between two guides which it moves up and down as it revolves. As it accomplishes the downward movement of the fingers in a third of a revolution, it permits of a larger shutter opening than any other movement in general use. The Geneva, or Maltese Cross movement, has been used in camera construction, and while it gives a quicker downward pull of the film than the harmonic cam, it has several disadvantages which preclude its use. In the Geneva movement the downward draw of the film is accomplished in about an eighth of a revolution, but, as this movement has four bearing surfaces which are liable to wear unevenly it has not found much favor as a camera (Continued on page 1714)