International projectionist (Jan-Dec 1947)

image and, where short-focus lenses are used, the "keystone effect" and difficulties in focussing. The dimensions of the standard soundfilm aperture, 0.825 X 0.600, have been selected in order to produce a picture image whose height is three-quarters its width when the effective projection angle has it maximum allowable value (approximately 15°). To determine apparent projection angle: Method 1. Measure the angular tilt of the projectors by sighting with a pocket level held against a protractor. Another way is to draw a straight line across a large smooth sheet of wrapping paper. Hold this up against the machine so that the straight line crosses the plane of the optical axis of the machine somewhere along its length. Then make the line true to the horizontal with the help of a level. Next, place points on the paper opposite the optical axis of the machine— a point opposite the lens, another opposite the aperture, and another opposite the arc. Remove the paper and connect the points with a straight-edge. With a protractor measure the angle at which the two straight lines intersect. Repeat the process several times for each machine to get the most accurate value. Method 2. Measure the level distance from the screen to a point directly below the projectors. Measure the projection throw by any of the methods previously given. Then use the following rule or formula : Rule: To find the apparent projection angle, divide the level distance from the screen to a point directly below the projectors by the throw. The result will be the cosine of the apparent projection angle. By consulting the trigonometric table find the angle which most nearly corresponds to this cosine. This angle is the apparent projection angle, itself. Formula: d is the level distance from the screen to a point directly below projectors; A is the throw: (Note: The mathematical abbreviation cos-1 means the inverse of the cosine, or "anticosine". More specifically, it means the principal value of the inverse cosine. The small minus one is not an exponent. Another way of expressing cos-1 is "arc cos".) To determine the effective projection angle: Measure the angle of tilt of the screen from the vertical by means of a protractor and level. (A perfectly perpendicular screen will, of course, have a 0° tilt, and in such a case the apparent and effective projection angles are identical. Remember — the screen tilt, if any, is toward the projectors.) Use the following rule or formula. Rule: To find the effective projection angle, subtract the angle of tilt of the screen from the apparent projection angle. Formula: #ap is the apparent projection angle; S is the angle of tilt of the screen from the vertical: $ — $ap. — S To determine the width of the picture: The "median width" of the picture image is the width across the point where the optical axis intercepts the screen. For all practical purposes this may be taken as the middle of the screen. (Even where the projection angle is quite steep, the point of interception lies only a short distance above the center of the screen.) Median width is specified on the data sheet because where steep projection angles and short-focus lenses are used, the picture will be slightly under normal width at the top of the screen, and somewhat wider than normal at the bottom. Method 1. Measure the width of the picture image across the middle of the screen directly by means of a tape measure. Screen masking should be ignored when measuring picture width. Method 2. Rule: To find the width of the pic FORM VI. OPTICS, PICTURE SIZE, SCREEN DATA Theatre City State Date _ Revised Theatre Circuit Chief Projectionist 1. LENSES Remarks Number of projectorsMake & Type lenses 3. SCREEN DATA E. F. of lenses Has normal focus been changed by altering the distance between front and back elements? : "f" value of lenses Diameter of barrels Size of screen Size of clear areaMake of screen Type surface Perforations Shims or special jackets Special effect lenses: Make & Type No. Date purchased Date resurfaced Angle of screen tiltMasking : Material used Details Specifications Corners square or rounded? Remarks 2. THROW AND PICTURE SIZE Is projector installation centered relative to auditorium? . . Projection distance Apparent projection angleEffective projection angleAperture dimensions : Type apertures , Width Height __ Screen stretcher & support, details How is screen flown for stage presentations? Auditorium dimensions : Length Width Balcony height from floor: First Second Third Square or rounded corners? Picture image dimensions : Median width Actual height Remarks Vertical extenuation 10 INTERNATIONAL PROJECTIONIST • October 1947