Slow motion

This article is about the filmmaking technique. For the song see Slow Motion.

Slow motion is a technique in filmmaking whereby time appears to be slowed down. Typically this 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.

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

Athletics diving, to demonstrate skill and style, or to recapture a key moment in a ball game. These scenes are often 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 aura or suspense or naturally 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.

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.

A VCR may have the option of slow motion playback, sometimes at various speeds; this can be applied to any normally recorded scene. The concept of slow motion may have existed before the invention of the motion picture: e.g. a Japanese form of theatre (Noh) employs very slow movements.

Japanese director Akira Kurosawa was a pioneer using this technique in his 1954 movie The Seven Samurai. American Sam Peckinpah was another classic lover of the use of slow motion.

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

OvercrankingTimeline.png|thumb|441px|right| For the purposes of making the above illustration readable a projection speed of 10 frames per second (fps) has been selected, in fact film is usually projected at 24 fps making the equivalent slow motion 48 fps. ]]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.

$Perceived Speed = \frac{Projection Speed}{Camera Speed}$

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 tiny crossfade 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.

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How slow motion works

Overcranking

Time stretching

See also