International projectionist (Jan 1959-Dec 1960)

Glass Aperture Pressure Plates To Control Film Buckle? Not infrequently in articles appearing in IP it is stated that large film used for more brightness and better definition is not preferable because the extremely hot light causes the film to flutter and it is impossible to maintain focus. Has it ever been tried, or even considered, to have two Pyrex pressure plates to hold the film flat when the film is stationary, then to let up on the pressure when the film is being pulled down? — GEORGE L. BAKER, Wilmington. California. HPHE suggested application of glass 35-mm film to buckle, thermal and ■1 aperture pressure plates for mo mechanical conditions remaining the tion-picture projectors revives the peren same, nial film-buckling problem and at once brings to mind the use of glass pressure plates in the gates of 35-mm filmstrip projectors. Each frame is exposed in a filmstrip projector for a considerable length of time, however, and no pressure-release mechanism is ordinarily employed to separate the glass plates during frame shifts. The use of pressure-plate aperture Plate Use Impractical glasses in filmstrip projectors is neces The suggested use of glass pressure sary to avoid the loss of focus which plates in the gate of a wide-film motionwould result from the rather severe picture projector to prevent annoying buckling of unsupported film. Even buckling is unfortunately impractical, when the light source is only a tungsten T . , , , , , b-,1 f , ,. . HI UIC 111 oL pidtC, 1 11C ii Id 35 UldLGo ulb of a tew hundred watts rating, u . • j . r ., c, . , ■ , , , , , /-i would retain and transfer to the film enough heat is absorbed by the film • , u , . r , . ™ £1 , . r r i a considerable amount of heat. Ihe film emulsion after a tew seconds exposure „ i ■ .. ti u u x* j „ . . r r emulsion might thereby be softened or to snap it out ot focus. u ■ c ■ ,, r r even scorched, oecond, the presence ot Similar buckling "excursions" of film the piates wouid prevent air-cooling of take place in a powerful motion-picture tlie film at the aperture. Third, the glass projector in a mere fraction of a second. surfaces would quickly collect oil and ^ust wn'cn would show up on the Film Width Big Factor screen, marring the picture. Fourth, the The width of the film is undeniably vibratory motion of the plates, necesa very significant factor in film buck sary for releasing the film during the ling. To be specific, it is the unsupported intermittent pulldowns to prevent scratchwidth of film between the gate runners ing, would create an intolerable amount that is important. Now, all films from of noise. 8 to 70 millimeters in width have the It should be noted that the "unsupported width" in large-frame horizontal VistaVision film is exactly the same as in regular 35-mm film, hence the superiority of horizontal VistaVision over 70-mm Todd-AO as regards film flutter and buckling. same thickness (approximately 0.006 inch for triacetate stock), hence the susceptibility to buckling is practically a function of the unsupported width of the film divided by its thickness. We can therefore calculate the relative "buckle susceptibilities" of the various film sizes: Other means are available for minimizing film buckling, though no known method can prevent it completely. These are the pulsed air blast described by Willy Borberg in the October 1952 issue of IP (page 14 et seq.), and the curved gate discussed in Borberg's later studies titled "Gate and Shutter Characteristics" in IP for June 1958 (p. 13 et seq.), and July 1958 (p. 10 et seq.). The modulated air blast is capable of reducing film buckle to about 40% of normal, on the average; but the effectiveness of this method is often unpredictably capricious and disappointing. Completely dependable, however, is the curved film gate (available for Simplex projectors) which reduces buckle to about 75% of normal. The curved gate has the added advantage of equalizing focus across the frame from one side to the other. At arc currents in the 60 — 80 ampere range, and with conventional straight film gates, 35-mm triacetate film exhibits an in-and-out of focus excursion distance of from 0.020 to 0.030 of an inch measured at the center of the frame over the aperture. Buckling is usually more severe at appreciably higher arc currents, especially if neither heat filters nor forced air-cooling of the film is employed. Actual tests prove that a curved gate reduces 0.030 inch of buckle, which would normally occur in a straight gate when arc current is about 80 amps, to approximately 0.023 of an inch. Even so, and under identical conditions of heat and gate curvature, 70-mm film will have a buckle excursion distance in the neighborhood of 0.060 inch. This is admittedly extreme, and it has the bad effect of forcing the large frames into a "pincushion" shape which destroys sharp focus toward the screen sides. The writer's conclusion: 35 millimeters represent the maximum practical film width for motion-picture projection prints. Todd-AO 70-mm prints evidence no advantages whatever in regard to image clarity. The What, Why and How of Chromatic Aberration FILM "UNSUPPORTED" REL. BUCKLE WIDTH ; 8 mm 16 mm 35 mm 70 mm : WIDTH SUSCEPTI BILITY 0.2 inch 33 0.4 inch 67 1.0 inch 167 2.0 inch 333 It can be seen from the above that he buckling susceptibility of 8-mm film is negligible, while that of 16-mm film is less than half (40%) of the tendency of standard 35-mm film to buckle under the impact of heat. On the other hand, the 70-mm film used in the Todd-AO process has fully twice the tendency of White ray Glass prism GREEN BUJE^ VIOLET •**■ -White Crown Flint glass glass ACHROMATIC LENS Here is expressed graphically the cause and correction of chromatic aberration. Note that a single ray of white light passed through a glass prism is bent toward the thicker part of the prism both when entering the glass and when leaving it. Of course, the "shorter" violet and biise rays are refracted more than the "longer" red rcys. INTERNATIONAL PROJECTIONIST FEBRUARY 1959 13.