Archive for March 18th, 2014
The CHDK firmware for Canon point-and-shoot cameras includes a USB remote trigger feature that depends on simply applying +5 V to the USB power leads, which is exactly what happens when you plug an ordinary USB cable into a PC.
Chopping up a spare cable, adding header pins, attaching a bench supply, and whacking the pins with clip leads showed that the camera takes quite a while to haul itself to its feet and click the shutter:
- Manual mode: preset shutter & aperture
- Manual focus
- Focus assist off
- Image stabilization off
- AF guide light off
- Red eye reduction off
- Flash enabled, medium intensity, precharged
Turning the flash off slightly reduces the delay, at least judging from when I hear the shutter click while watching the trace trundle across the screen. I may have forgotten to turn something else off, but I doubt it’ll get an order of magnitude faster.
I’d hoped to synchronize an outboard flash with the shutter, but watching a few traces shows that the time from trigger to flash isn’t very consistent; maybe 100 ms jitter, more or less.
The CHDK motion-sensing script works and is “lightning fast”, but it turns out that lightning strokes actually glow for tens to hundreds of milliseconds, so my 1 ms xenon flash will be over and done with by the time the script reacts and opens the camera shutter.
Other ways to synchronize an outboard flash with the shutter:
- Fire the outboard flash from the camera flash, with the camera flash inside a shield
- Use an absurdly long shutter time with the camera & objects inside a very, very dark enclosure
- Use the CHDK motion detection script, but blink an LED into the lens to trigger the shutter, then fire the xenon flash to expose the image
Choice 1 has positive synchronization to the camera shutter, but the shutter delay jitter means the flash won’t happen after a fixed delay from the triggering event. Maybe it’s not as bad as I think.
Choice 2 requires that the shutter stay open longer than the maximum delay jitter, so the flash will happen at known time after the triggering event. I like that, but not the dark enclosure part.
Choice 3 depends on the timing jitter of the script, which should be on the order of a few tens of milliseconds. A shutter speed of 1/25 s = 40 ms might be Good Enough.
This obviously requires a bit of Arduino fiddling…