there’s a film strip that has a bunch of frames. there’s a device that pulls the film from the reel so that each frame is briefly positioned in front of a light that shines into the frames, thereby projecting it onto a screen. So why don’t we ever see the film on the screen in states where a frame is partially in front of the light but not at the ideal center location where the full frame is in view? According to my logic, we should see a bunch of transitional states, not steady, perfectly centered frames. How does this work?
There is a shutter in front of the film gate that rotates at just the right speed so that the light is blocked off from the screen while the film is in transition. Since the film is projected at 24 frames/second our “persistence of vision” compensates so that it appears to be in continuous motion.
I believe movies are shown at 72 frames per second*, the human eye is incapable of seeing things flash at that speed. Flourescent lights flicker at 60 cycles per second and to our eyes the light is always on. You can make a simple tool that can verify the blinking of lights and movies. Make a round disc about 12 inches across. Cut four 1/4 inch slots every 90 degrees, make the slots about 4 inches long. Tack the disc loosely to a pencil or dowel so it will easily spin. Spin the disc and look through it at a flourescent light, movie screen, even your computer moniter or a TV screen. At random times you will see things the human eye normally can’t see, your catching it out of phase. Get the disc spinning at just the right speed it is possible to catch everything out of phase.
*This is a silly wild ass guess, please correct me if I am wrong.
There is a mechanism in the projector, called a Maltese cross, which ensures that the shutter only opens when the frame is completely centered. The timing of the mechanism is controlled by the sprocket holes on the edge of the film.
Boy I guess I was off. I know the disc thing works with movies though, when I in 6th grade the whole class went to see Gone With The Wind and we all had our spinner cardboard discs. The guy running the theater must have wondered what was going on, when the movie started here was a bunch of kids spinning cardboard discs while oohing and aahing. 15 minutes into the movies everyone was bored with the cardboard discs, 30 minutes into the movie everyone was bored by the movie, 45 minutes into the movie we all left, everyone was getting rowdy and pissed off our teacher.
Sorry, but this is incorrect. Movies are shown at 24 frames per second, although as Q.E.D.'s link points out, each frame is illuminated twice by the shutter, resulting in a rate of 48 “flashes” per second.
TV, on the other hand, is shown at the rate of 30 frames/second (more precisely, 60 fields /second with each field comprising 1/2 of the frame using alternating lines). That is why if you shoot a movie of someone watching TV (or a computer screen) the frame rate is out of sync and you see black bars scrolling up the screen. In professional productions they can work around this, but if you see news footage or amateur videos of people watching TV it is a problem.
The shutter helps tremendously, but the main reason is the pause on each frame. Even if you completely remove the shutter, you won’t see transitional images. You’ll notice a slight vertical streaking effect and might think the picture is a bit out of focus because of it, but you wouldn’t be aware of the pulldown unless it was running very slowly. It just goes by too fast, you’ll only be able to make out the image when it stops, and it always stops in the same place on every frame.
There were many toy 35mm projectors in the '20s, 16mm in the 30s, and handheld 8mm viewers well up into the '70s that didn’t have any shutters at all. They relied entirely on the afterimage of the paused frame to overpower the quick movement of the passing film, and they do work.