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Electronic Guidebook for High-Speed Flash Photography

revised 12-00

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Activity 2. Controlling flash duration

 

Equipment needed:

  • Vivitar 283 flash unit
  • 4 AA batteries or SB-4 AC adapter for flash power
  • 120-V AC extension cord
  • 100-kΩ variable resistor

Background: An important factor to control for high-speed photography is the flash duration, that is, the amount of time the flash of light lasts. Typically, one wants the shortest possible burst of light. Figure 1 shows the intensity of the light vs. time for a full discharge. The intensity is not constant but rather increases from 0 to a maximum in about 0.3 ms.1 It then decreases more gradually over a period of about a millisecond. This is too long for high-speed photographs. However, if the flash unit is used in the automatic exposure mode,2 the duration can be decreased to as little as 30 µs.3 Such a discharge is represented in Figure 2. Note that the time scale is expanded by a factor of 50 over that of Fig. 1. The intensity scale is expanded by about a factor of 10, pointing to the fact that decreasing the duration of the flash also decreases the brightness. A 30-µs burst will be quite dim compared to a full discharge. However, there will still be sufficient light for photography.

 

The photographs below the graphs illustrate the effect of flash duration.

Figure 1

FLASHGR1.gif (1787 bytes)

Figure 2

FLASHGR2.gif (1937 bytes)

hs01-06r.JPG (5130 bytes) hs02-28r.JPG (4412 bytes)
A golf ball was dropped onto a black table for this photograph.  The sound trigger on the left detected the sound of the collision and discharged the flash unit.  The flash was set in manual mode so that the duration of the discharge would be a maximum.  Note first that the ball is on the rebound, moving up.  The top of the ball shows significant blur. This effect, called afterglow, results from the gradual decay of light intensity as shown in Fig. 1.  Note also that the trailing side of the ball is sharp.  Here the intensity from the flash is rising steeply to a maximum. For this photograph, a golf ball and a rubber ball were dropped together beside a post.  The sound trigger is not visible this time.   The automatic-exposure circuit of the flash unit was used in order to shorten the flash duration to approximately 1/10,000 second.  There is no blurring this time and the printing on the ball is visible.

In order to obtain the briefest flash of light, one needs to arrange for the intensity of the reflected light to be as great as possible.4   This activity will show how to do that.

hspeqp03.JPG (11221 bytes)Note the small, circular window on the front of the auto-thyristor module. (See diagram to the left.) This window allows light to reach a light detector5 inside the module. Light must reach this detector in order for the flash unit to operate in its automatic mode. In the M (manual) setting, the window is completely covered, and the flash unit produces its brightest and longest flash of light. For each of the colored settings--yellow, blue, red, purple--a different neutral-density filter is rotated in place over the light detector. These filters reduce the intensity of light reaching the light detector. The darker the filter, the brighter and longer the flash of light will be.6 For high-speed photography, the most transparent filter would be used in order to obtain the shortest flash duration.  These filters can be seen in the photograph of a disassembled auto-thyristor module below.

hspeqp16.JPG (7119 bytes)

Disassembled photosensor module shows colored bands corresponding to automatic exposure ranges.

hspeqp15.JPG (4934 bytes)

The view through the disassembled module shows the neutral-density filters that would be rotated over a photocell to select the different exposure ranges.

  1. Which of the colored settings gives the shortest flash duration? Which gives the longest? In order to find out, aim the flash unit at a distant wall, making sure to keep the window over the light detector uncovered. Placing the flash unit in each of the colored automatic modes in turn, discharge the flash unit. Which mode gave the brightest flash? Which gave the dimmest?  Remember that the brighter the flash, the longer it lasts.
  1. Another way to change the amount of light reflected to the light detector is to change the distance from the flash unit to the subject. Try this by discharging the flash when it is very close to the wall and then again when it is as far away as you can get it. Connect an extension cord to the SB-4 to get greater distance.   Use the yellow automatic mode. (You should have found that the yellow mode gives the shortest flash of light.) Did you get the brighter flash when the unit was closer or farther from the wall?  Explain why this result is to be expected.

What does this tell you about where you should place the flash unit for taking high-speed photographs?

  1. A third way to change how much light is reflected to the light detector is to change the background. A light background will obviously reflect much more light than a white one at the same distance. Try this by discharging the flash unit first toward a white wall and then toward a black curtain at the same distance as the wall. What did you find out?
TipFrequently in taking high-speed photographs, a dark background is preferable so that the subject of the photograph will stand out.  Since this works against a decrease in flash duration, the selection of flash mode (#1 above) and placing the flash unit closer to the subject (#2) are the primary means of decreasing flash duration.
  1. In order to review what you have done so far, what three things can you do in order to obtain the shortest burst of light from the flash unit?
TipIn addition to minimizing flash duration, using the yellow automatic mode will maximize battery life. Therefore, whenever possible when using batteries, be sure the flash unit is placed in the yellow mode and that the flash head is in the 90° position so that the reflected light reaches the photocell in the auto-thyristor. Be sure, of course, that nothing covers the detector.
  1. The Vivitar 283 has a feature that makes it possible to conveniently adjust flash duration. Note that the auto-thyristor module can be removed. Pull it straight out, away from the flash unit. This will expose an arrangement of five holes, as shown below. The two holes on the right (as seen with the flash unit upright and facing you) are the connections for the light detector. By inserting the leads of a variable resistor into these two holes, you can control the flash duration (and brightness) directly. Try this now with the 100-kΩ variable resistor.
hspeqp06.JPG (9231 bytes) hspeqp08.JPG (8367 bytes)
Auto-thyristor module pulled out, revealing socket of 5 holes Variable resistor has replaced auto-thyristor module
hspeqp07.JPG (8403 bytes)TipOften in high-speed photography, one wants the shortest possible flash duration. In that case, one need not use the variable resistor described above.  Simply short across the two holes in the sensor socket with a stiff wire. A paper clip works fine, as shown in the diagram to the right.
  1. There is at least one more way to influence flash duration with the Vivitar 283.   This is with the use of a sensor extension cord.  This cord, which is described in Appendix B, allows the sensor to be placed a little more than 4 feet from the flash unit.  One could, for example, place the sensor in a location facing the flash unit so that the direct light of the flash unit reaches the photocell.  The intensity would therefore be much greater than if the sensor were used in the normal way to detect reflected light.

Notes

  1. The abbreviation, ms, stands for millisecond. The prefix, milli-, represents thousandths. Thus, 1 ms = 0.001 s (second).
  2. In the automatic mode, the light reflected by the subject is detected by a photoresistor in the auto-thyristor module on the front of the 283. A small capacitor in series with the photoresistor charges through it. When the voltage across the capacitor reaches some predetermined amount, the flash discharge is quenched.
  3. The abbreviation, µs, stands for microsecond. The prefix, micro-, represents millionths. Thus, 1 µs = 0.001 ms = 0.000001 s.
  4. This results because the resistance of the photoresistor (see Note 2) decreases as the light intensity increases. The lower the resistance, the quicker the series capacitor will charge.
  5. This is the photoresistor referred to in Note 4.
  6. One would use darker filters in order to use smaller apertures for greater depth-of-field. For high-speed photography, however, the greater flash duration that results is usually undesirable.

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