At 22:08 5/4/99 , Siddiq wrote:
>There was a post about someone's shots getting ruined by leaving the
>camera on the FP position--what exactly does that do? I know it's Focal
>Plane, but what difference does it make where the collar is? Thanks,
>Siddiq
I've read most of the other replies as I just got back home and downloaded
a bazillion e-mails from this list . . . most active mailing list I have seen.
[back to topic]
The FP position *is* for "Focal Plane." With standard strobe and even
flash bulbs, focal plane shutters have limitations on the shortest shutter
speed that will sync with strobe flash that leaf shutters do not have. The
problem is that the curtains can only travel so fast. Shorter than a
certain shutter speed (about 1/60th with most but the newest and more
expensive), the closing curtain is released before the opening curtain is
fully open. At speeds shorter than this point you get a slit smaller than
the width (for horizontal shutters) of the frame of film traveling across
the film gate. At speeds of 1/1000 or 1/2000, this is a rather narrow
slit. Some of the newer cameras can X-sync the focal plane shutter at
1/125th or 1/250th because their curtains travel so fast. Many still
require 1/60th or slower. Even some of the oldest leaf shutters from the
1950's can X-sync at 1/500th which is about the maximum speed for a leaf
shutter.
For a strobe or standard flash bulb to expose the frame properly using a
focal plane shutter, the flash must fire *after* the opening curtain has
fully traveled but *before* the closing curtain has been released.
Otherwise you will get a portion of the frame exposed correctly and the
rest rather dark. This speed on most OM SLR's is 1/60th second (or slower)
for X-Sync which will fire the flash when the film gate is fully open. For
flash bulbs, the sync is typically 1/30th or slower as the bulbs have a
much longer peak duration than a strobe. In spite of this *normal* flash
bulbs (M or F) do not have a peak duration long enough for both opening and
closing curtain to fully travel.
The FP sync solves the problem of using focal plane shutters at faster
shutter speeds along with special FP type flash bulbs, or nowadays a
dedicated flash with an FP mode position. *Both* the camera body and the
flash *must* have this mode (as in the OM-4T with F280, or my Contax IIIa
CD with M26 flashbulbs circa 1954). In a simplified explanation, the
camera body triggers the flash slightly before the opening curtain is
released in this mode so that the bulb is at peak output when it opens.
The flashbulb has a longer peak duration than normal flashbulbs (M or F) to
allow the closing curtain to fully travel before it burns out. How a
strobe such as the F280 does this is by firing many times, each firing only
releasing a portion of the energy stored in flash's charged capacitor (?).
This ensures that as the slit travels across the film gate, the entire
frame is exposed to the same amount of light. The guide number goes down
as the shutter speed increases beyond the camera body's fastest X-sync in
FP mode because the slit across the film gate gets narrower.
-- John
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