Yes, I think your OM thinking is rusty. OM shutters (except for the
oddball OM-2000) run their cloth shutter curtains from side to side and
not top to bottom as with metal curtain shutters like the OM-2000.
I've never thought much about curtain travel time but think it must be
the same as the max synch speed. Consider the shutter curtain actions
at OM max sync speed of 1/60 second. In order for the full film frame
to be exposed to very high speed electronic flash the first shutter
curtain must be fully open and the second curtain must not yet have
started to close. That means the first curtain must have gotten fully
open in 1/60 second and the second curtain is now immediately starting
to close. If it travels in 1/60 second also the entire frame will have
been exposed for 1/60 second but there was a very brief time when the
entire frame was exposed for flash.
Vertical metal blade shutters have faster sync times because they have a
shorter distance to travel.
I dunno but I think TTL/OTF exposure above sync speed must be determined
entirely from the shutter curtain before it opens and not OTF. Since
shutter speeds above sync speed are formed by a traveling slit the
timing of the release of the second curtain must be known at the time
the first curtain starts moving. Otherwise, exposure determination
couldn't be done across the entire frame and one couldn't change
exposure once the first curtain started traveling... a varying slit
width on a single exposure would be unwelcome.
Chuck Norcutt
On 5/13/2014 7:10 PM, Mike Gordon via olympus wrote:
raises the question of how fast the OM shutter
is above the 1/60 sync speed??? (i.e time to totally expose the film
from top to bottom) It must be very fast after the magnet signaled to
release the 2nd curtain is released as never recall "rolling shutter"
artefacts.
I should know this, but must be rusty thinking about OM. A related
question is the TTL/OTF metering. Above sync speed perhaps is hybrid of
OTF and off the curtain until exposure quench point reached for 18%??
Below sync speed must be 100% OTF.
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