There has been a lot of discussion of how the Super FP mode is triggered and
stopped by the 4Ti etc.
For example Tom asked :
>The question is, will the F280 flash in Super FP mode without any special
>signals from the camera aside from "Start." After all, the flash amount
>doesn't vary or stop during the exposure.
Looking at the 4Ti service manual there is a fair bit of detail on what
happens: As might be expected the flash starts flashing at the moment the
first curtain starts to move and stops after the second curtain closes.
The auto mode operates exactly as it normaly would in daylight. (with no flash)
That is, the second curtain is released when the integrated light reflected off
the shutter+film reaches the correct exposure level but the flash continues on
until the second curtain closes. This results in total super FP flash durations
of between ~19ms to 40ms depending on light level. Cutting the light off at the
shorter times obviously conserves battery power and speeds recharge. But the
range of times show the flash at best saves only half the power in FP mode.
The flash in FP mode seems to use **only the two new contacts** ( the ones not
on the T series flashes). The Right hand contact is termed "CLK" and the Left
"D.B."
Just before the first shutter blind runs an analog timer is held reset and the
CLK contact to the flash is held low.
The flash is triggered as the curtain releases, by the CLK pin going high
(transistor switch turns off and it floats high).
The timer also starts then and is used to delay when the light integrator
starts integrating the light off the film/shutter curtain. Once the integrator
integrates enough light it starts the second curtain and **after a delay** to
account for the shutter travel time the flash is stopped.
Looking at the timing diagram it appears the D.B. contact is held low only
during the period from first shutter release to release of second shutter. The
flash must still be held on for a further duration of the second curtain
travel. This means the F280 must have a **timer of it's own** to sustain the
flash output until the second curtain has closed. This easy to do as the
curtain travel time is constant, but of course the flash must accomodate the
slowest shutter member of the OM's that are designed to work with the FP mode.
The OM4Ti curtain travel time is of the order of almost 11ms. Giving rise to a
theoretical ~ 22.5 ms minimum flash duration.(11+11+0.5ms) a little more than
claimed in the service manual.
In summary: CLK terminal starts the flash on the positive going edge while DB
stops the flash on positive going edge, but the F280 must add a delay before
flash switch off.
Regards,
Tim Hughes
TimHughes@xxxxxxxx
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