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Slow motion or slowmo is an effect in film-making whereby time appears to be slowed down. It was invented by Austrianmarker August Musger. Typically this style is achieved when each film frame is captured at a rate much faster than it will be played back. When replayed at normal speed, time appears to be moving more slowly. The technical term for slow motion is overcranking which refers to the concept of cranking a handcranked camera at a faster rate than normal (i.e. faster than 24 frames per second). Slow motion can also be achieved by playing normally recorded footage at a slower speed. This technique is more often applied to video subjected to instant replay, than to film. High-speed photography is a more sophisticated technique that uses specialized equipment to record fast phenomena, usually for scientific applications.

Slow motion is ubiquitous in modern filmmaking. It is used by diverse directors to achieve diverse effects. Some classic subjects of slow motion include:

  • Athletic activities of all kinds, to demonstrate skill and style.
  • To recapture a key moment in an athletic game, typically shown as a replay.
  • Natural phenomena, such as a drop of water hitting a glass.


Slow motion can also be used for artistic effect, to create a romantic or suspenseful aura or to stress a moment in time. Vsevolod Pudovkin, for instance, used slow motion in a suicide scene in The Deserter, in which a man jumping into a river seems sucked down by the slowly splashing waves. Another example is Face/Off, in which John Woo used the same technique in the movements of a flock of flying pigeons. The Matrix made a distinct success in applying the effect into action scenes through the use of multiple cameras, as well as mixing slow-motion with live action in other scenes. Japanese director Akira Kurosawa was a pioneer using this technique in his 1954 movie Seven Samurai. American director Sam Peckinpah was another classic lover of the use of slow motion. The technique is especially associated with explosion effect shots and underwater footage.

The opposite of slow motion is fast motion. Cinematographers refer to fast motion as undercranking since it was originally achieved by cranking a handcranked camera slower than normal. It is often used for comic effect, time lapse or occasional stylistic effect.

The concept of slow motion may have existed before the invention of the motion picture: the Japanesemarker theatrical form Noh employs very slow movements.

How slow motion works

There are two ways in which slow motion can be achieved in modern cinematography. Both involve a camera and a projector. A projector refers to a classical film projector in a movie theatre, but the same basic rules apply to a television screen and any other device that displays consecutive images at a constant frame rate.

Overcranking

[[Image:OvercrankingTimeline.png|thumb|441px|right| For the purposes of making the above illustration readable a projection speed of 10 frames per second (frame/s) has been selected, in fact film is usually projected at 24 frame/s making the equivalent slow motion 48 frame/s.]]Overcranking is a technique that achieves slow motion by photographing images at a faster rate than they will be projected. Normally great care is taken to ensure that the camera will record sequential images at the same rate that they will eventually be projected. When a faster camera speed is selected, the projection rate remains the same. The result is that photographed movement will appear to be slowed down. The change in speed of the onscreen image can be calculated by simply dividing the projection speed by the camera speed.

\mathrm{Perceived\;Speed} = \frac\mathrm{Projection\;Speed}\mathrm{Camera\;Speed}

Most video cameras do not allow the operator to select a frame speed faster than the projection speed. For this reason, overcranking is sometimes referred to as film slow motion because it is most often achieved with film cameras. Digital overcranking is currently rare.

Time stretching

Frames marked with an X must be fabricated.
The second type of slow motion is achieved during post production. This is known as time-stretching or digital slow motion. This type of slow motion is achieved by inserting new frames in between frames that have actually been photographed. The effect is similar to overcranking as the actual motion occurs over a longer time.

Since the necessary frames were never photographed, new frames must be fabricated. Sometimes the new frames are simply repeats of the proceeding frames but more often they are created by interpolating between frames. (Often this interpolation is effectively a short dissolve between still frames). Many complicated algorithms exist that can track motion between frames and generate intermediate frames that appear natural and smooth. However it is understood that these methods can never achieve the clarity or smoothness of its overcranking counterpart.

Traditionally, frames were duplicated on an optical printer. True frame interpolation can only be done digitally.

Simple replication of the same frame twice is also sometimes called half-speed. This relatively primitive technique (as opposed to digital interpolation) is often visually detectable by the casual viewer. It was used in certain scenes in Tarzan, the Ape Man, and critics pointed it out. Sometimes lighting limitations or editorial decisions can require it. A wide-angle shot of Roy Hobbs swinging the bat, in the climactic moments of The Natural, was printed at half-speed in order to simulate slow-motion, and the closeup that immediately followed it was true overcranked slow-motion.

A VCR may have the option of slow motion playback, sometimes at various speeds; this can be applied to any normally recorded scene. It is similar to half-speed, and is not true slow-motion, but merely longer display of each frame.

In action films

Slow motion is used widely in action films for dramatic effect.

In Broadcasting

Slow-motion is widely used in sport broadcasting and its origins in this domain extend right back to the earliest days of television, one example being the European Heavyweight Title in 1939 where Max Schmeling knocked out Adolf Heuser in 71 seconds.

In instant replays, slow motion reviews are now commonly used to show in detail some action (photo finish, Football goal, ...). Generally, they are made with video servers and special controllers.

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