From: "Bob Blakely"
> I'm curious. What can the flash know of rear curtain sync timing save
> that given it by the camera's shutter? If nothing, what's to set on
> the flash? What difference could it possibly make?
This is just a guess, but I know the AF-500FTZ has a switch position for
rear curtain sync.
What I think is somewhere in Pentax's dedicated flash connection,
there's a pin that gets a signal (V+ or ground??) when the shutter
curtain opens and loses that signal when the rear curtain starts to
close. Say something like this:
Curtain Curtain
|Opens |Closes
__------------___________ V+
OR
--____________----------- ground
It'd be easy to build a trigger circuit to detect the state changes and
switch the flash trigger to either the leading edge change or the
trailing edge change.
Thinking about it now, a standard hot shoe could just go to ground for
the duration of the curtain being open, so you don't even need the
dedicated pin to trigger rear curtain sync; just design your rear
curtain trigger circuit so it will flash when the ground circuit opens
again.
For a lark, I tried the AF-500FTZ with a K-1000, and it doesn't do rear
curtain sync even with the flash switch set in rear curtain position, so
I think Pentax does use a dedicated pin.
And, I'll bet it's the 4th pin in approximately the 2 o'clock position
that permits rear curtain sync, since Pentax had TTL Auto-flash with the
LX and Super Program and AF-280T, which I'm pretty sure uses the two
additional pins at 5 o'clock and 7 o'clock.
The AF-500FTZ manual says rear curtain sync only works with the PZ-1
[PZ-1P], which I expect was the only camera with that 4th dedicated pin
at the time the manual was written, because AF-500FTZ rear curtain sync
does also work with the *ist-D.
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