I wrote:
: but the manual for the F280 says that the flash duration in FP mode varies
from
: 1/50 to 1/25 sec with the OM-4Ti and OM-3Ti, and from 1/70 to 1/30 sec with
the
: OM-707. I've never seen the real explanation, but at least the speed
difference
: between the cameras must have something to do with the speed of the curtains
: (the OM-707 has a faster curtain). And I can only guess that the slower speed
: (1/25) is used when the camera is set to manual, with shutter speeds of 1/30
or
: longer.
After sending this message I realized that there _is_ some additional info about
the mysterious flash duration in FP mode: the document 'Applications of slow and
fast streak recording cameras' ( http://www.rit.edu/~andpph/text-streak.html )
that describes a research of the F280's FP mode on the OM-4T for Modern
Photography, using a streak recording camera (some kind of rotating drum with
film).
It appeared that at least with the camera in Manual Mode the FP duration changes
continously in the shutter speed range 1/60 to 1/2000 (and also provides some
insight in the accuracy of the manual shutter speeds. Aaargh 1/1400 instead of
1/2000?):
SET MEASURED SLIT FLASH
TIME TIME WIDTH,mm DURATION
1/60 1/64 >36 1/27
1/125 1/130 26 1/35
1/250 1/260 13 1/41
1/500 1/550 6 1/45
1/1000 1/890 3.5 1/46
1/2000 1/1400 2 1/45
Surely with the camera in Auto mode the change in flash duration (and thus
stroboscopic frequency) will be similar. After all, the F280 specifications also
say the duration varies when used with the OM-707, and this camera doesn't even
support Manual Mode.
So the operation scenario for the OM-4Ti with the F280 in FP mode would be:
- the flash starts to emit stroboscopic light pulses
- the first curtain starts to move
- the camera measures the light (the mixture of available light and flashlight)
reflected from the curtain, and determines the final shutter speed
- the flash duration / stroboscopic frequence is adjusted according to this
shutter speed, to make it more effective for the shorter speeds.
H@nz
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