Shutter Speed
Every video camera boasts a high speed shutter, a term borrowed from still photography.(Still cameras have shutters that open to allow light to fall on the film or CCD). Video cameras don't have shutters. When the video camera is switched on, light falls on the CCD continously, but the CCD doesn't constantly record an image. Recording occurs only when the video camera supplies the CCD with an electrical charge, in other words when you press record.
Fields and frames- Like a television screen, the CCD is made up of a number of horizontal lines. In some countries like australia this number is 625(PAL), in others like USA is 525(NTSC). The video camera only supplies a charge to half of these lines at a time. It alternates between supplying a charge to the odd and even number lines. Each of this half images recorded by the CCD is called a field. Two fields, one of odd number lines and the other of even, are interlaced to form a frame. Each field is a snapshot of the action at a given moment. If the shots are played back fast enough, the brain sees a moving image.
Fields per second- In australia where the standard for television screens have 625 lines(PAL), the CCD records 50 fields per second. With TV's that uses 525 lines(NTSC), video cameras record 60 fields per second. With a 1/50 sec shutter speed, each charge supplied to the odd or even lines lasts for 1/50 sec. The CCD records an image continuously, as 50 fields are recorded a second.
With a high speed shutter, such as 1/1000 sec, each charge is supplied for only 1/1000 sec. As there are still only 50 charges per second, there are gaps of 19/1000 sec between each field when no image is recorded.
When you play back a moving subject recorded with a high speed shutter, these gaps cause it to move like someone under a strobe light.
With a high speed shutter, each field is blur-free, because most subjects hardly move in so short a time. Played back in slow motion each frame can be seen clearly. This is ideal for analysing action, such as your golf swing or tennis serve. |
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