Conventional Film projectors do not advance from one frame to another by continuous movement of the reels. What is the reason for the intermittent motion and blocking of light during change of frame in conventional film projector?what is the mechanism used in the projector to achieve these two processes?

  • $\begingroup$ What are the little square holes for on the film stock? $\endgroup$
    – Solar Mike
    Aug 13, 2020 at 4:24

1 Answer 1


if the film was fed smoothly past the lens and bulb, your eye wouldn't see a series of pictures- all you'd see would be a continuous blurry smear.

So instead, the projector pulls one frame of film into the optical gate, stops the film there for a brief instant while the light shines through it, and then pulls the next frame of film in, stops it, etc.

This presents your eye with a series of frozen image frames in fast succession. Your brain then stitches those frozen frames together into a continuously-moving image that looks real.

The reason the light is interrupted during the frame change is if it wasn't, then you'd see a blurry smear as the film was being indexed between successive frames and the image quality would be very bad. Cutting off the light whenever the film is moving between frames prevents this effect.

The light is turned on and off with the use of a continuously rotating disc. The light shines through an opening in the disc which coincides with the instant that the frame being held still. The light is cut off by the disc for all times the film is being indexed to the next frame.

The mechanism that indexes the film resembles the piston-and-connecting-rod arrangement in an old steam locomotive. This is an example of what mechanical engineers call a four-bar linkage.


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