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For a variety of reasons, I recently got interested in using a combination of Live View / EFSC and flash exposure with my Canon T1i camera. As far as I know, Canon does not support this combination. Connecting a flash to the camera disables EFSC and the camera reverts to mechanical first curtain. This behavior seems odd, since the camera also supports second curtain flash sync, but that's the way it is.
Anyway, it finally occurred to me that I could get around this limitation by just not connecting the flash to the camera, thus letting the camera run in EFSC mode, and triggering the flash by some separate mechanism. This seemed pretty straightforward, so I built the following little piece of electronics:
I noticed in breadboard that there seemed to be some variation in the timing, just listening to the sound sequence of "zing, pop, click" corresponding to start of exposure, flash going off, and shutter closing. But I figured a bit of variation was to be expected, so I just made the delay adjustable and figured I'd tune it after I got everything boxed up.
After I got the flash delay timer boxed up and started using it, I noticed that every once in a while the flash would trigger prematurely, so that actually the whole exposure occurred after the flash was gone. Black frame, not nice. Clearly a glitch in the electronics -- or so I thought.
At that point I was getting hungry for solid data, so I decided to set up some instrumentation. I hooked up an oscilloscope watching the flash trigger line, synced by the shutter trigger line, and pointed the camera at the oscilloscope so I could see when the picture was being taken.
When I first turned everything on, the timings looked great -- shutter delay about 170 milliseconds, almost perfectly repeatable, flash trigger right on schedule as set. No trouble at all dialing the flash trigger right into the middle of a 1/15 second exposure.
But then I realized I had the camera set to normal mode, not Live View.
So I switched it over into Live View. That changed the picture a lot!
It turns out that my electronics are working as designed.
The problem is that in Live View, the camera has a large variation in shutter delay between receiving a trigger and actually taking a picture. In one series of 100 frames, "large variation" meant that the shutter delay varied from a low of 70 milliseconds to a high of just over 500 milliseconds. Here are some examples at 100 ms/div:
The bottom line is that using this approach, I cannot use less than about 0.6 seconds flash delay and 0.6 second exposure time. Using a shorter flash delay or a shorter exposure time means that every once in a while either the camera will finish before the flash fires, or the flash will fire before the camera starts.
I think this will still be useful on occasion, but it doesn't work nearly as nicely as I had hoped.
I tried a variety of image sizes and formats, with the camera connected and not connected to a computer. No obvious difference in behavior. The key issue seemed to be only whether the camera was set to run in Live View or not.
--Rik
