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Tools - Video Cameras

 

Imaging

 

High-speed phenomena can be captured with modern, consumer-grade video cameras. This is possible, because these cameras use imaging elements, called CCDs (charge-coupled devices), that have the following property in common with film cameras used with electronic flash: the entire image is captured at once on the light-sensitive element (either film or CCD). This may seem obvious, but there are common situations in which the image is scanned across the sensitive element. One such situation is described next.

 

si-02-03.jpg (15042 bytes)A single-lens reflex camera (SLR) typically uses a focal plane shutter. For the photo to the right, the back of the camera was removed in order to show the shutter. Another camera took a flash photograph of the shutter in the act of opening. (Exposure time was 1/500 s.)  When an SLR takes a photo at fast shutter speeds, the shutter curtains actually form a slot which sweeps across the film in order to expose it. (This occurs at shutter speeds which are faster than the X-synch or flash synchronization speed for the camera.) If fast motion is being photographed, the moving subject will be changing position as the film is being exposed. As a result, distortion of the image will occur.

 

The series of photographs below demonstrate this distortion and how to avoid it. The subject of the series is a black disc with 4 bright lines spaced 90° apart. The photo to the left shows the stationary disc. The disc was next set into rotation at about 50 cycles per second. For the middle photograph, the camera shown above, a Konica FT-1, was set for an exposure of 1/1000 second. (In this case, the slot would be half as high as in the photo of the camera above.) The disc was photographed under ambient lighting. The distortion in the rapidly moving lines is apparent. For the photo to the right, the disc was photographed with the same camera using electronic flash at 1/125 s exposure time. At this exposure, the shutter is completely open long enough for the flash to discharge completely. In this case, the flash duration was about 1/1000 s. Note that the lines, while blurred, are not curved. With shorter flash durations, the blur could be reduced, and the disc could be made to appear stationary as in the first photo.1

 

Stationary disc Moving disc photographed with SLR, 1/1000 s exposure, no flash Moving disc photographed with SLR, 1/125 s exposure, electronic flash

 

The key to obtaining undistorted, sharp images of high-speed subjects is for the entire sensitive area of the film to be exposed at once for a very short period of time. A CCD video camera can be used for this purpose when the subject is illuminated by electronic flash to freeze the motion.2 

 

Notes

 

1. For more discussion on the distortion created by focal-plane shutters, see this article: "Observations Through a Moving Slot," L.M. Winters, The Physics Teacher, 376 (1994).

2. Prior to the modern CCD cameras, video cameras scanned the image across the sensitive element of a video tube. Such cameras produce distortions similar to those described above.

 


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