"On auto exposure, the duration is always ~1/25. This is close to the full time
from 1st curtain open to 2nd curtain close at 1/60. The curtains take 1/80
(12.5ms) to go across the frame, so for 1/60th (16.666ms) exposure the shutter
travel is 16.666 + 12.5 = 28.66 ms or 1/29 second. For 1/2000, .5 + 12.5 =
1/77. Of course, on auto, you don't know the final shutter speed when you start
the exposure and you don't want to run out of light before you finish."
I hadn't thought of that. Right! It takes 1/60 s for the first shutter to open,
then another 1/60 s for it to close (at an effective shutter speed of 1/60), so
the flash would have to fire for at least 1/30 second. If the shutter speed is
higher, then the closing of the second shutter curtain can terminate the flash.
This (finally!) explains the variation in Super FP flash time.
In any case, I'm going to call Olympus tomorrow and try to get a full
explanation of all the details of how the F280 works, along with the design
theory and goals.
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