| Posted by: Neil Midkiff at January 16, 2006, 5:44 am | | Topic: why does zombie movies have to be about zombies taking over? Forum: groupsrv | | I wrote: Quote:Silent films had used almost all the width of the film between the perforations, and had very thin horizontal lines between each frame, every four sprocket holes, making the image almost three-quarters of an inch high and almost an inch wide. In the process of explaining the various frame apertures and ratios I forgot to mention one thing I realized while looking up various numbers. You'll often see a description of 35mm film as having six perforations per inch (for instance, the Wikipedia article on 35mm film). It isn't true. Six sprocket holes and six intervening spaces add up to one and an eighth inches. It's probably easiest to remember it as four sprocket holes in 3/4 of an inch (the distance from the top of one frame to the top of the next), or as a sprocket hole every 3/16 inch. 35mm still-camera film uses the same perforations as movie film; I think the Leica was the first to take advantage of that format. So if you have a strip of 35mm negatives or the tag end from a box of slides you can easily verify this with a ruler. Of course the Leica format runs the film horizontally (like VistaVision movies) and the still frame size is 24 x 36 mm, advancing the film 8 sprocket holes (1.5 inches) between shots. 36 mm comes out to about 1.42 inches, leaving a small unexposed line of 0.08 inches between the frames. Or you can confirm it with sound film speed calculations. At 24 frames/second, sound film moves at 90 feet/minute. Multiply by 12 inches/foot to get 1080 inches/minute. Divide by 60 seconds/minute to get 18 inches/second. Divide again by 24 frames/second to get 0.75 inches/frame. Divide by four perfs/frame to get 0.1875 inches/perf, which is 3/16 inch per perforation. Now one thing I can't explain is why the width of the film stock is specified in millimeters and the sprocket hole spacing is in inches. Did Edison or Dickson specify their films from Eastman originally as 1 3/8 inches (34.925 mm) wide? Or are my sprocket-hole numbers not quite exact in inches, and really some metric measure that comes out to very near 3/16 inch? It turns out that's the case today. As usual, the Kodak web site gives the right numbers: modern 35mm print film has perfs every 4.75 mm, which is 0.1870 inches. http://www.kodak.com/country/US/en/motion/support/h1/sizesP.shtml I'll probably still remember the number as 3/16 inch. Anyway, the 1/6 inch figure that I had often read is clearly wrong. -Neil Midkiff |
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