Dependence of Discharge Curves on Flash Charge Time

In February 1997, Jeremy Stein, an amateur photographer and physicist working in Albuquerque, New Mexico, measured the light output of the Canon 430 EZ and 380EX electronic flash units. This page shows the results for the dependence of flash output on the duration of flash charge time.

Jeremy writes:

I just finished testing (retesting) the 430EZ, and learned a lot more about its behavior. This thing is SMART!

I noticed while I was shooting the lower power levels that the duration of the pulse at any given power level seemed to vary. I thought that might be due to not waiting long enough for full charge, so I ran a very simple test. At 1/8 power, I waited a long time after the red ready light came on, then I fired eight shots at one second intervals and recorded them all on the same scope trace with no change in settings. The traces clearly shows that the flash unit lengthens the flash duration as its peak level falls off. I believe that the area under the light intensity versus time curve is held to about the same level by adjusting the flash time. This means, by the way, that stating what the duration of the flash is based on the power setting is iffy at best, and is a variable in any case. Note that all these results were obtained with no camera attached to the flash; the 430EZ decided itself what durations to use for the successive flashes.

On another day, the tests were repeated for the 380EX:

The series consists of data taken on a Canon 380EX, attached to an ElanIIE body, and flashed by the camera at various times after the red flash ready light came on. The times were: when the light came on, 5 seconds after, 10 seconds after, 15 seconds after, and 30 seconds after. The change in peak intensity is from 1.9 volts fired at the instant the ready light came on to 2.8 volts when fired 30 seconds after the ready light came on. Note also that the half peak level time for the first shot was about 950 microseconds, while the same measurement on the shot fired 30 seconds after the light came on was about 1150 microseconds. Nothing else changed during this series, so the scope data differences are due solely to level of charge. The batteries were nearly new, but I expect that the timings would be different for older or newer batteries. The data should probably serve more as a warning that it is best to wait a while after the flash says it's ready, although I suspect the difference will never be seen in the pictures.

[Ed. note: "volts" means volts as measured from the output of the light-to-electrical signal converter. We expect to have a linear correspondence between the "volts" and light levels, but the numerical relationship between the two is not known to us.]

This image shows the output from the flash fired right after the red "flash ready" light came on. It is quite similar to those measured earlier.

If one waits 5 seconds after the "ready" light is on, the output is much more powerful, though. I would expect almost twice the Guide Number from this output compared to the previous one.

Waiting for 10 seconds adds even more power. A small "plateau" starts to form near the peak, showing energy reserves in the capacitor.

If you wait for 15 seconds after the "ready" light comes on, the capacitor has enough charge to keep an almost steady current through the xenon for about 250 microseconds.

Increasing the charge time to 30 seconds after "ready" seems not to introduce any furher interesting changes.

Thus the observation also stated in the Flash FAQ that full GN is achieved long after the "ready" light comes on is also true for Canon flash units.

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