Slow motion
Slow motion is a technique in film-making whereby time appears to be slowed down. It was invented by Austrian 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 slower. The technical term for slow motion is overcranking, referring to the concept of cranking a handcranked camera faster than normal (i.e. faster than 24 frames per second). 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 Japanese 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
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.
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
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.
See also
Motion picture terminology
High speed camera
Time-lapse
Bullet time
External links
Interesting High-speed Video Clips
High-speed video clips, info and links
Lucid Movement, high-speed daily video blog
JackCabbage: Overcrank on the EX-1
JackCabbage: Overcrank and Undercrank on the HVX
YouSlow.com, Where everything is slow..
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