Movie Makers (Jan-Dec 1949)

426 NOVEMBER 1949 8 MM „ulTc SOUHO 16 MM and Motion Picture { Service WRITE FOR PRICES DEPT. M GEO.W.COLBURN LABORATORY, Inc. 164 N. WACKER DRIVE, CHICAGO 6, ILL. MOVIE AND SLIDE TITLES STILL AT SAME LOW PRICES! Same titles formerly distributed by Bell & Howell — now sold direct. Large variety backgrounds available. No charge for tinting film Amber I WRITE FOR free illustrated brochure and samples TITLE-CRAFT, 1022 Argyle St., Chicago 40, III. GREATEST TELE-LENS BARGAIN! 5'/2 INCH F-3.5 COATED S32.5Q POSTPAID Sharp AAF I38MM, our fixed focus "C" mounts, for distant shots of birds, game, sports, etc. • Color corrected. Same as above in our focusing "C" mounts $39.50 • De Luxe Model with case $47.50. These 5'/a power lenses for I6MM Bolex, BH, Revere, Keystone, etc. Bring subject 5'/2 times closer! CENTURY PRECISION SPECIALTIES CO. 11960 Wilshire Blvd., Los Angeles 25, Calif. NEW 8mm 50 ft S2.75 8mm 100 ft 5.50 SELECTION OF UNUSUAL FILMS FOR HOME ENTERTAINMENT 16mm 100 ft. ..$ 6.00 16mm 200 ft... 11.50 cpcci 8mm — 16mm. Unusual FILM subjects and CAMERA BARGAIN CATALOG. Write for it TO-DAY. Friart Camera Exchange, Inc. 140 LIBERTY ST.. N. Y. 6— Dept. B-ll— CO. 7-5172 I ACL MEMBERS: The ACL has on hand a ! limited number of 8mm. black and white j ACL leaders. Normally priced at 50c, they j are available at 35c each as long as the ! supply lasts. Send orders and remittances ' to the Amateur Cinema League, 420 Lex I ington Avenue, New York 17, N. Y. ■ Want a real buy in 16 mm SOUND FILMS? Take advantage 'f the I.C.S. 1949 CLEAR CLEARANCE ANCE SALE of 16 mm sound films — Features, Comedies, Educationals, H Cartoons, Novelties, etc. ■ '/-'■.^^^B rjui giant list I "A" today stating machine* you own (Make and model). INSTITUTIONAL CINEMA SERVICE, INC. 1560-M2 Broadwoy, New York 19, N. Y. FILM FILM C0L0RCHR0ME (In a class by itself) 100 ft. 16 mm $6.50 100 ft. double 8 mm $6.50 25 ft. double 8 mm $2.25 I lighl onto Weston peed of 8) N n pectrum colors, bul our own tint ;i"'l tone tern 1009! Improvement over black and white. BLACK & WHITE (Weston 8) 100 ft. 16 mm $2.00 100 ft. double 8 mm $3.00 25 ft. double 8 mm $1.75 PRICE ON ALL FILM INCLUDES PROCESSING A.i.i iau m In Calif. Order Now.' Rl TLITCD 'C 1715 N. Mariposa Ave I V_ n I t K 9 Hollywood 27, Calif. Stroboscopes are easy [Continued from page 412] DESIGN CAN BE SIMPLIFIED We could now draw up a strobe disc with 60 black segments and 60 white ones, and when this disc appeared to stand still under the pulsations of the neon lamp, the projector would be showing at exactly 16 frames per second. The reason for this is that the segment movement has the same frequency as the flashing light. But drawing up 60 black segments of a small circle is a boring task at the best. Let's see if the job can't be simplified. We have stated just above that the movement of the alternating segments has the same frequency (120) as the flashing neon lamp. It should be true, then, that any number of segments which is a simple multiple or fraction of this frequency should function in the same way. To state it differently, the 60 black segments divided or multiplied by a whole number will give the same results. This gives us our chance for simplification. For there is no use drawing more segments than necessary. We find that 30, 20, 15, 12, 10, 6, 5, 4 or 2 will go into 60 a whole number of times. But too few segments on the disc would not create clearly the desired optical illusion. It is a good idea, therefore, to try several combinations before making a final choice. To begin with, I discarded 20 and 10 as possibilities. For these figures will work equally well on a disc designed for 24 frame per second projection. I finally decided that 15 black segments were the least I should use. Another 15 could easily be added later, and another 30 after that, if it proved desirable. HOW TO DESIGN THE DISC Fig. 1 shows how I laid out my disc, and will give you one to cut out for yourself, if your conditions are the same as mine. With a sharp lead in my compass, I drew three concentric circles on a card. The inner one was only to limit the length of the segments. The middle circle was the diameter I wanted for my disc. The outer circle was for construction purposes only, its greater diameter producing more accuracy in the next steps. Now, with my compass set at a little greater span than the radius of the construction circle, I stepped off around the circumference from "A." It took four tries, readjusting the compass span a little each time, before I was able to land exactly at the starting point in five equal steps. In the same manner, these fifths were then divided into thirds. The result was 15 equal divisions of the circumference. Some stroboscope discs are drawn with fan-shaped segments creating equal white and black spaces, as in Fig. 2. That is excellent, if the full number of segments indicated by the formula are used. In such a case the construction circle should be divided into twice as many parts as there are black segments. However, good results can be obtained if only half as many segments (as the number indicated by the formula) are used, provided that the glow lamp is positioned so that one electrode supplies more light than the other. Any fewer than half the correct number of segments, drawn in the fan style of Fig. 2, would be entirely unsatisfactory. I decided to stick to 15 straight lines. In the case under discussion, optimum clarity is obtained by having the thickness of the lines about equal to one eighth of the space between them. You will notice in Fig. 1 that each black segment lies on an imaginary line connecting the center of the two inner circles with a division of the construction circle. These lines indicate the design treatment of a line-type disc instead of the fan type. Fair success will be had by drawing the segments (or lines) in pencil, but India ink is much better. The disc can be attached to the takeup shaft in any number of ways. Even chewing gum will do in a pinch. I will leave that up to you. One method appeared in July, 1948, Movie Makers. Another was described in the May, 1949, issue. DISC FOR TESTING KNOB But, you say, your projector cannot accept a strobe disc on the sprocket hub. Such is the case, for example, with many of the Ampro and Bell & Howell projectors, in both 8mm. and 16 models. The next step then is to determine the possible pattern for a disc on the hand testing knob. First comes the formula, which is a different one from that used in sprocket disc design. It read as follows: i 2 x f JN equals r The values here are: N equals the number of black segments; f equals the frequency of the current cycle, and r equals the number of revolutions of the hand testing knob in passing the desired number of frames per second that is to be your projection speed — i.e., 16, 18, 24 fps. For an example let us take, say, the Ampro 8 projector. We find that its hand testing knob makes 24 revolutions in passing the standard of 16 frames. Substituting in the formula, we get the following: I N equals 120 ~24~ N equals 5 black segments The actual designing of such a disc