Originally Posted by Delphinus
Unfortunately it's still driving the camera rather than the flash. Even with a very fast camera a 37 millisecond shutter lag the pellet would be long gone by the time the shutter opened and you'd be limited to 1/250th exposure as well if you used a flash. If the camera was driving the flash you'd also have to deal with the intentional delay that's built into the camera software that would fire the flash after the shutter opens to ensure the sensor see's the whole cycle. You'd also have to deal with the delay caused by the iPhone app. This probably needs to be an analogue trigger to get the kind of speed of response needed. It may even need to be triggered by the sound of the sears or hammer rather than muzzle blast.
When you shoot on flash the camera assumes that the flash is the primary light source and will open the shutter for as long as possible to ensure it gets all of the light available from the flash. Shooting above synch speed ends up with some wierd effects due to the shutter not exposing the whole frame at the same instant in time. The only way to get a really fast shutter speed is to have a shutter that's effectively a slit that moves down or across the frame.Anything that triggers the camera is unlikely to work.
Driving the flash is the only way you could stand any chance of capturing the pellet, although I suspect that even taking this approach it'll be a 2-3 inch streak rather than clearly defined pellet emerging.
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