A slow motion effect is obtained by playing filmed material back at a rate slower than the rate which was used to film it, thereby staggering the frames and causing the action to appear slower, even though it actually occurred in real time. There are several ways to achieve this effect and it is a common film making technique seen everywhere from sports broadcasts to art films.
One way to create this look is to overcrank the camera being used to film, meaning that the camera films at a more rapid rate than it would normally. When the resulting film is projected with a projector using a regular rate, the overcranked film will be seen in slow motion. It is also possible to interpolate frames into film which has already been produced to slow down the action, creating slow motion. This technique is useful because it can be utilized in post-production, after the film has already been shot and someone decides that they want to see a slower version.
In films and television shows, a conscious decision is usually made to use slow motion ahead of time, and the camera will be overcranked at the appropriate time. In situations like sports shows, the slowed-down version is usually achieved by interpolating frames into film shot at normal speed. This type of slow motion is often used to highlight a moment of interest, or to replay fast action at a speed which people can actually see. At some sports games, one camera may overcrank continuously to make slowed-down instant replay available.
Frame interpolation can be tricky. Modern film makers have the advantage of computer programs which can be used to morph the two surrounding frames to create a new frame which will mesh seamlessly when the film is played back. Historically, the interpolated frames were sometimes made by copying one of the neighboring frames, which could create a choppy or strange look which often attracted the attention of critics. Blank frames were also sometimes inserted, which could cause the film to flutter in a rather distracting way when it was viewed.
The technique was developed by August Musger, an Austrian film maker who is unfortunately remembered for little other than this contribution to the art of filming. Musger worked in an era when filming and playback speeds were different than they are today, and he might be astounded to see the remarkably smooth and even look of modern slow motion.