For the OM-2N atleast: as diaphragm lever goes up to close lens down,
metering circuit turns on (old OM-2, this was present along with micro
switch which closed as mirror started to move. Part of an OM-2 overhaul is
to remove the microswitch.)
First curtain should fire when mirror gets within about 1/8" of hitting
mirror foam. First curtain is always released before second. If not,
curtains would leave starting gate in an over lapping position, there would
be no slit, no exposure. While there is a tiny electromagnet to hold the
second curtain open, there is no solenoid per se.
When the diaphragm firing lever / mirror come back down, auto metering
circuit turns off. If mirror locks up for some reason, auto metering circuit
turns off in 3.5 minutes, timed out by P board in bottom of camera (2N
only). If mirror locks up in OM-2 and is left that way, batteries will
run down.
John Hermanson
Camtech Photo Services, Inc.
21 South Lane,
Huntington, NY, 11743-4714,
631-424-2121, www.zuiko.com
_________________________________
----- Original Message -----
From: "Julian Davies" <julian_davies@xxxxxxxxxxxxxx>
To: <olympus@xxxxxxxxxx>
Sent: Sunday, September 26, 2004 7:54 PM
Subject: [OM] Re: OM Metering
>
> There are two almost independent things going on in an OTF OM
> The mechanical thing:
> Mirror rises
> Meter switches on to integrate off the first curtain
> slight delay 1
> First curtain is released
>
> The electronic thing:
> Meter sees enough light integrated and switches off
> Causing the Solenoid to drop
> slight delay 2
> Thereby releasing the Second curtain
>
> The only point of interface is that the mirror NOT the first curtain
> switches on the meter. Evidently there are two delays in here which have
> to
> be accurately aligned.
>
> For a highest speed run, the second curtain is released before the first
> starts to move. All the metering takes place during delay 1
> At any speed the meter is switched off before the second curtain runs.
>
> If it wasn't switched off, what would you like it to be doing? Making tea?
> It has no power to control anything except the release time of the second
> curtain.
>
> The two delays are the reason why an OM can NEVER flash sync at 1/60th in
> auto OTF. The shutter assembly is capable of it, as is the meter, but the
> two windows of opportunity overlap for less time than 1//60th requires.
> The
> flash sequences are:
>
> Mirror up
> Meter on - sees blackness
> Delay 1
> First curtain start
> First curtain end
> X sync contacts closed
> Fire flash
> Meter integrates
> Solenoid drops - back emf is the quench signal
> Short delay 2 - this is the one that prevents the 1/60th
> Second curtain starts
> X Sync contacts open
> Second curtain finish
>
>
> The real benefits of OTF are in flash control and long exposure. For short
> exposure it makes not a jot of difference. the only real reason for doing
> the metering OTF rather than direct is that direct metering the middle of
> the light path would be bloody hard without casting a shadow or using a
> beam
> splitter in front of the mirror. That would just be expense for no
> benefit.
>
> The patterning on the first curtain is primarily for show. The centre
> weighting on an OM2 is defined by the crossing of the two receptors, not
> by
> the pattern. Single receptor OMs do it by simple trigonometry and a single
> element lens in front of the SBC.
>
>
> Julian
>
>
>
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