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Electronic Guidebook for High-Speed Flash Photography

revised 12-00

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Activity 8. Photographing waveforms on an elastic band

 

Equipment needed:

  • Vivitar 283 flash unit
  • 4 AA batteries or SB-4 AC adapter
  • Tape recorder sound trigger
  • SLR camera and film
  • Tripod
  • Large rubber band or elastic cord

Suppose an elastic cord is stretched end-from-end, pulled to the side and released.   What shape will it take?  The answer is surprising to most people.  If you want to find out the answer for yourself, don't click here.   Instead, read on.

 

With a sensitive sound trigger, a snapped elastic cord can be observed almost immediately after its release. A sound trigger made from a tape recorder works well. Go here for instructions on how to construct the trigger.

 

Use a large rubber band or a length of elastic cord. If you use a rubber band, cut it so that you can stretch it end-to-end. Hold the microphone of the tape recorder near the point where the band will be plucked. Aim the flash unit at the band. When the band is plucked and the flash discharges, the initial waveform can be observed. Move the microphone farther from the release point to observe the waveform at a later time.

 

TipWhen photographing small subjects against a dark background, the flash unit's automatic-exposure circuit is influenced more by the light reflected from the background than from the subject.  When the background is dark and far away, the intensity of the reflected light at the flash unit's photocell will be low, causing the flash duration to be larger than desired.  The problem can be reduced by hanging a background (even a dark one) within about a meter of the subject.  Alternatively, one could control flash duration using other methods such as those described in Activity 2.
TipA problem in taking photographs with a sensitive sound trigger is that the click of the camera shutter may actuate the trigger prematurely, giving unwanted exposures. To avoid this, hold a hand over the camera lens while opening the shutter.

 

The next activity will introduce you to another trigger that can be used to capture the plucked cord and many other phenomena.

 


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