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Re: [OM] F280 vs 3rd party Fill Flash

Subject: Re: [OM] F280 vs 3rd party Fill Flash
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Tue, 15 Jun 1999 00:40:33 +0000
At 00:32 6/15/99 , Terry wrote:
>
>How well do these F280's work at oh say 1/1000s are they reliable?
>Thanks
>Terry
>

Perhaps a description of how focal plane shutter works at higher speeds
will clear this up.  The curtains in a focal plane can only travel so fast
across the film gate (area where the film gets exposed).  With the OM's
(don't know about the 2000) at shutter speeds of 1/60 and slower, the
opening curtain completely travels before the closing curtain is released.
If the X-sync fires the flash just as the opening curtain completes its
travel, the flash will expose the entire frame.

At shutter speeds faster than 1/60 (1/125 and up) the closing curtain is
released *before* the opening curtain has completely traveled.  This means
at the faster shutter speeds you have a slit traveling across the film
gate.  The curtains themselves travel no faster . . . the faster the
shutter speed the narrower the slit becomes.

How the F280 "FP" syncs at 1/125 and up is it gives out a rapid series of
light bursts . . . so fast you cannot really perceive the individual bursts
. . . it just looks like a long flash.  This series of bursts begins
*before* the opening curtain begins to travel and *continues* until after
the closing curtain completes its travel.  This is what makes it different
from X-sync and why you need not only an F280, but electronics in the
camera as well, such as an OM-4T, that will FP-sync the flash.  My OM-4
does not have these electronics and hence cannot use the FP-sync feature in
an F280.

With flashes such as the F280 in FP-sync mode, as the shutter speed
increases (at 1/125 and faster) each point on the film receives less light
from the flash because the slit gets narrower and receives fewer of the
series of light bursts.  This is why the guide number of the flash goes
down as the shutter speed gets faster when operated at 1/125 and faster in
FP mode.

It is also why, to finally answer your question, if the F280 works
(FP-syncs properly) at 1/125, it will work at 1/1000, and even 1/2000
(assuming your shutter is working correctly . . . providing a constant
width slit across the film gate).  Heck if the camera could reliably
produce a slit narrow enough for 1/10,000 second it would still work . . .
because the curtains are still traveling at the same speed they did when
they open and close at 1 second exposure.

BTW, this is *not* a new technology.  My 1954 Contax IIIa CD switches to
FP-sync at shutter speeds of 1/100 to 1/1250th.  In those days you had to
use special long-duration flash bulbs (FP) that put out a constant amount
of light during the opening and closing curtain travel.  Normal flash bulbs
(M or F) didn't hold peak light output long enough.  The F280 (and other FP
mode strobes) simulate the long duration bulbs with the rapid series of
light bursts.

Hope this explains better how an FP flash works at high shutter speeds and
why FP-sync timing is completely different from X-sync timing.

-- John

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