By: Andrew Seltz
Our ability to perceive motion when viewing a sequence of still images is due to a phenomenon called ‘persitence of vision.’ There is a threshhold of frames per second (fps) where the human mind stops distinguishing between individual frames and begins to see motion (around 16 fps.) Film and video do not record and play back at the same number of frames per second.
Film Frame Rates
Film is recorded at 24fps. The shutter on the projector runs at 48 frames per second (displaying each frame twice) to reduce the perception of flickering light intensity for the viewer.
The 24 fps rate was locked in as a standard with the introduction of sound. Silent films tended to run at 16 to 18 fps, but optical sound tracks required the film to move faster. Sound also demanded that film play back at consistent frame rates because changes in speed altered the pitch of the sound.
In the early silent days there could be a considerable variation in the recording and playback speed of the film. Early film cameras were hand cranked, so the operator’s sense of timing affected the recording frame rate. Also, some exhibitors would speed up the films they showed in the theater so they could have more screenings per day and increase their revenues.
Video Frame Rates
Video runs at 30 fps (technically 29.97fps, but usually rounded up for general reference.) For technical reasons which I won’t try to explain here, each frame of a video is actually broken up into 2 fields in a process called interlacing. One field contains the odd lines of an image and the other contains the even line. The fields are recorded one after another. Perceptually the image is refreshing at 60 fields per second. This higher speed playback rate is partially responsible for the more ‘real’ looking nature of video.
When it was first established, the playback rate of video was tied to the frequency rate of our country’s electrical system. This is why the frame rate of European television is 25 fps.
HD And The New Format Explosion
The advent of digital video and high definition have added a bit of confusion to the frame rate discussion. It is now possible to record and playback video at a variety of frame rates. However, even in this new situation, frame rates tend to be either 24 fps, 30 fps, or 25 fps (the European standard video rate.) The difference now is that, with many cameras, you can pick the frame rate you want rather than have it forced upon you by the medium.
Transferring Film To Video
Movies, commercials, and television programs which are shot on film get converted to video’s frame rate using a process called 3:2 Pulldown. The film is slowed down by 0.10% and then the frames are transferred in a pattern that mixes together frames so that every 4 film frames results in 5 video frames.
The first video frame is 2 fields taken from the first film frame. The second video frame is 2 fields taken from the second film frame. The third video frame is a mix of film frames 2 and 3. The fourth video frame mixes film frames 3 and 4. The fifth video frame is 2 fields from the fourth film frame.
1 2 2-3 3-4 4
This pattern repeats throughout the transfer. Usually you can’t see any difference, but slow panning shots and title crawls will exhibit a slight juddering effect.