Lesh said:
I would guess that if you can see the 20ms main flash, the 5ms pre flash would be visable. I am taking those numbers from your scope image.
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You would be the first one. I think what is happening is that they are to closely spaced. As an example you can not distinguish the individual frames in a 16mm movie that is projected a 16 frames per second.
The spacing is 140 ms (1/7 second). I hadn't given thought to the spacing between the flashes. Given the spacing between the flashes, I would guess that you could discern a difference between a signle flash, and the preflash / flash, although it would not appear to be a second flash.
You did say that "The preflash(es) on all cameras are so fast that you can not see them." I disagree, I am sure I could see a 5ms flash.