Time-lapse
Time-lapse photography is a cinematography technique whereby each film frame is captured at a rate much slower then it will be played back. When replayed at normal speed, time appears to be moving faster and thus lapsing. Processes that would normally appear subtle to the human eye, such as motion in the sky, become very pronounced. Time-lapse is the extreme version the cinematography technique of undercranking.
Some classic subjects of timelapse photography include:
- cloudscapes and celestial motion
- plants growing and flowers opening
- fruit rotting
The technique has also been used to photograph crowds, traffic, and even television. The effect of photographing a subject that changes imperceptibly slowly, is to create a smooth impression of motion. A subject that is changing quickly already is transformed into an onslaught of activity.
The first major usage of time lapse in a feature film was Koyaanisqatsi. The non-narrative film was composed almost entirely from time-lapse of clouds, crowds, and cities. Countless other films, commercials, TV shows and presentations have included time-lapse. For example, Peter Greenaway's film A Zed & Two Noughts featured a sub-plot involving time-lapse photography of decomposing animals and included a composition called Time-lapse written for the film by Michael Nyman.
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How Timelapse Works
Film is normally projected at 24 frames per second (fps). Meaning that 24 images appear on the screen every second. Under normal circumstances a film camera will record images at 24 fps. Since the projection speed and the recording speed are the same the images onscreen appear to move normally.
However if the film camera is set to record at a speed slower than 24 fps it will still be projected at 24 fps. Thus the image on screen will appear to move faster.
The change in speed of the onscreen image can be calculated by simply dividing the projection speed by the camera speed.
<math> Perceived Speed = \frac{Projection Speed}{Camera Speed} <math>
so for example, a film that is recorded at 12 frames per second will appear to move twice as fast. Shooting at camera speeds between 8fps and 24fps is usually referred to as undercranking. Shooting at slower speeds is referred to as time-lapse.
The same principles apply to video and other digital photography techniques, however most video cameras do not have variable frame speeds.
Time-lapse can be achieved with normal movie cameras, though often the camera must be modified with a device called an intervalometer. The intervalometer regulates the motion of the camera according to a specific interval of time between frames.
Short Exposure vs. Long Exposure Time-lapse
In addition to modifying the speed of the camera it is also important to consider the relationship between the frame interval and the exposure time. This relationship essentially controls the amount of motion blur present in each frame and it is, in principle, exactly the same as adjusting the shutter angle on a movie camera.
Here is how it works:
A film camera normally records film at twenty four frames per second. During each 24th of a second the film is actually exposed to light for roughly half the time. The rest of the time it is hidden behind the shutter. Thus we normally calculate exposure time for motion picture film to be at one 48th of a second (1/48 sec often rounded to 1/50 sec). Adjusting the shutter angle on a film camera, can add or reduce the amount of motion blur by changing the amount of time that the film frame is actually exposed to light.
In time-lapse photography the camera records images at a specific slow interval such as one frame every thirty seconds (1/30 fps). The shutter will be open for some portion of that time. In short exposure time-lapse the film is exposed to light for a normal exposure time over an abnormal frame interval. So for example the camera will be set up to expose a frame for 1/50th of a second every 30 seconds. Such a setup will create the effect of an extremely tight shutter angle giving the resulting film a stop-animation or clay-mation quality.
In long exposure time-lapse the exposure time will approximate the effects of a normal shutter angle. Normally this means that the exposure time should be half of the frame interval. Thus a 30 second frame interval should be accompanied by a 15 second exposure time to simulate a normal shutter. The resulting film will appear smooth.
You can calculate the exposure time based on the desired shutter angle effect and the frame interval with the equation:
<math> Exposure Time = \frac{Shutter Angle}{360^\circ} \times Frame Interval <math>
Long exposure time-lapse is less common because it is often difficult to properly expose film at such a long period especially in daylight situations. A film frame that is exposed for 15 seconds will receive 300 times more light then its 1/50th of a second counterpart. (Thus it will be more then 8 stops over normal exposure.) A scientific grade neutral density filter can be used to alleviate this problem.
Related Techniques
External link
- Examples of time-lapse film
- Zeitcam.com Time-lapse animations of outdoor Web cams.