Moving Picture World (Jul 1916)

252 THE MOVING PICTURE WORLD July 8, 1916 Inquiries. QUESTIONS in cinematography addressed to this department will receive carbon copy in the department's reply by mail when four cents in stamps are inclosed. Special replies by mail on matters which cannot be replied to in this department, $1. Manufacturers' Notice. It is a rule of this department that no apparatus will be recommended editorially until the value of such appliances has been demonstrated to its editor. Standard Film Measurements. (Adapted from the standards computed by the engineers of the Bell & Howell Camera Company, Chicago, 111., and used by nearly all manufacturers of motion picture machinery.) The necessity for accuracy and uniformity in film perforation has been recognized from the beginning of the motion picture business. It is no less necessary that all motion picture apparatus be ^uilt in conformity with certain definite standards of construction. It should be possible to use film prepared by any perforator in any camera — to pass it through any printing mechanism without delay or difficulty — and to handle it successfully in any projector. That is to say, there should be set up a definite universal standard of film perforation to which tho film movement mechanism of all motion picture machinery should be made to conform. The adoption of such a standard would substitute order for confusion in the manufacture of motion picture machinery. It would go far toward perfecting the mechanics of the motion picture industry, thus opening the way for new artistic triumphs. And it would simplify the tasks and lighten the burdens of the manufacturer of films — the maker and exhibitor of motion pictures. The absence of definite universal standards for the construction of motion picture machinery made necessary the design and adoption of a standard film perforating gauge. The gauge is made of two bars of steel arranged in parallel and held equidistant from each other by parallel steel links. Between the bars are two strips of plate glass, one of which is fastened in the frame by ' means of a spring clip engaging the beveled ends of the glass plate while the other is held in place by the thumb. Each of the strips of glass carries on one of its margins a scale graduated to one full picture space for the purpose of determining the correct perforating gauge length. With this device it is possible to test the accuracy of perforations as to uniformity of spacing, marginal gauge and transverse alignment. When testing the uniformity of spacing the parallel bars are opened as wide as possible. The film is laid upon the lower glass plate and folded so that the celluloid surface is inside the fold. The upper strip of glass is put in position on the film, where it is held by the thumb (see cut). The bars are so placed as to contact the film margins snugly at all points, thus bringing the edges of the film in parallel. The gauge should then be held toward the light and the upper layer of film so adjusted over the lower that the smallest possible opening will appear in the perforation spaces. Any variations in the perforations creating an imperfect register will then be readily apparent. The standard film perforating gauge was adopted only after long experience in building of motion picture machinery and the handling of film. In setting up a standard of measurements it departs from Fig. 1.— Standard Film Perforation Test Gauge. earlier standards only where it is necessary to do so in order to take into account the physical changes to which the film is subjected. It is accepted as settled that the maximum shrinkage of motion picture film is .0937 inches per foot. Painstaking experiment warranted the conclusion — fully established by later experience — that a gauge length of 11.968 inches for 64 holes would insure the accuracy of perforation necessary to perfect results and at the same time make due allowance for shrinkage of the film. The following computation shows why we adopt 11.968 inches instead of -078 1-219 33 h-078 Fig 2. — Measurements of Standard Film. 12 inches as the standard for a perforating gauge measuring 64 holes: Assuming the outside diameter of the sprocket wheel in all standard projecting machines Xo be 15/16 inches or .9375 inches, then ; The diameter of the sprocket being .9375 inches The circumference of the sprocket is 2.94525 inches. As standard motion picture film has an average thickness of .0065 inches the pitch diameter of the sprocket will be found to be .9375 inches plus .0065 inches, or .944 inches. Pitch circumference is 3.1416 x .944 or 2.965704 inches. Circular pitch equal 2.965704 divided by 16 (the number of teeth to be found on the sprocket) or .1853 — plus inches. The standard perforating gauge being 11.968 inches for 64 holes and the maximum allowance for shrinkage of film being 3/32 inches or .0937 inches for 64 holes, therefore 11.968 inches less .0937 inches or 11.8743 inches is the average length of shrunken film measuring 64 holes. The pitch of the film or length per hole is 11.8743 inches divided by 64, or .18553 inches. Pitch of sprocket, .1853 plus inches Pitch of film, .1855 plus inches The drawing shows the measurements by which the gauge is established ; width of film over all 1.375 inch ; width of film over all on perforation 1.219 inches; distance (width) center to center of perforations 1.109 inch; distance (width) inside to inside of perforations .999 inch; width of perforation .110 inch ; distance (width) center of perforation to margin of film .133 inch; distance (width) outside of perforation to margin of film .078 inch ; length of perforation .073 inch ; distance (length) between perforations .114 inch; distance (length) center to center of perforations .187 inch. •Copyright, 1916, by the Chalmers Publishing Co. "The Illumination of the Field of a Photographic Objective." The above is the title of a reprint of a paper by H. C. Lord that appeared in Volumn LXXVI, No. 3, of the Monthly Notices of the Royal Astronomical Society. This paper, with its illustrations, is a valuable and instructive one, containing formulas that can be applied over a wide field. Mr. Lord advises that he will be glad to send copies of this paper gratis as long as the supply lasts. All that is necessary is to address a request to H. C. Lord, Director, Emerson McMillin Observatory, Ohio State University, Columbus, Ohio ; and, while Mr. Lord does not ask that it be done, our readers can hardly do less than enclose a stamp with their request in order that his kindness may not be made too burdensome. Professor Lord is the author of a book on "The Making of a Photographic Objective," which was reviewed in the department la6t February. The science of optics is the foundation and corner stone upon which rests the structure of photography, and no earnest student of the craft can ever learn enough in that too much neglected branch of his profession.