Strobe Photography: Drop Tests vs. Xenon Flash Energy

Tweaking the Arduino program to fire the LED 10 ms after the beam breaks, then fire the Xenon strobe 180 ms later produces this result:

Drop test - ISO 800 - 100 ms f8 - overexposure

Drop test – ISO 800 – 100 ms f8 – overexposure

Obviously, that’s far too much light: ISO 800, 1/10 sec, f/8, with the flash a few inches from the action. There aren’t many free variables:

  • Shutter must be open long enough to span the timing jitter
  • Aperture is already as small as it gets for good depth of focus
  • ISO speed may be too high
  • Flash intensity is fixed for a given capacitor

Throwing a shop rag over the flash helps a bit, capturing the ruler suspended in mid-air:

Drop test - ISO 800 - 100 ms f8 - cloth

Drop test – ISO 800 – 100 ms f8 – cloth

However, replacing the 250 µF electrolytic flash capacitor with a 1 µF film cap reduces the stored energy by roughly an order of magnitude and reduces the flash pulse duration to about 100 µs.

The bottom two inches of the ruler now have lighting from the flash, while the rest of the image looks pretty good in natural light:

Drop test - ISO 800 - 100 ms f8 - 1 uF

Drop test – ISO 800 – 100 ms f8 – 1 uF

It turns out that having the laser and photodiode beam-break sensor within the view (the white ring at the top) doesn’t work, as the CHDK motion detector will notice the red spot on the ruler and trigger the shutter before the LED (clipped to the right of the vertical steel scale) flashes.

Several more trials showed that the flash fires consistently, but (as expected) the shutter triggering has some jitter. In this case, the shutter remained open after the flash and captured a blurred image as the ruler continued to fall:

Drop test - ISO 800 - 100 ms f8 - tail

Drop test – ISO 800 – 100 ms f8 – tail

Here, the shutter closed immediately after the flash, eliminating the blurred tail:

Drop test - ISO 800 - 100 ms f8 - no tail

Drop test – ISO 800 – 100 ms f8 – no tail

Having the shutter close before the object reaches the bottom of the image is a Bad Thing, as it means the shutter triggered too early.

In both cases, the sharp image of the ruler overlays the blurred image captured in natural light. That’s more visible toward the top of the picture where the flash doesn’t reach very well.

I aligned the laser beam-break detector at 200 mm on the scale and the flash fired when the tip of the ruler was at 390 mm = 190 mm below the beam. The LED blinked 10 ms after the beam break and the Xenon flash fired at 180 ms; given all the vagaries involved, 190 mm is just about spot on the (revised) estimates.

But that background has got to go…

,