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Re: [OM] How does program mode on OM2S work?

Subject: Re: [OM] How does program mode on OM2S work?
From: clintonr@xxxxxxxxxxxxxxxx
Date: Thu, 22 May 2003 15:07:03 -0500
Paraphrasing the service manual (Page I-3), when the release button is
operated, it releases the aperture arm on the side of the mirror box.  This arm
begins to rise, which closes the main switch, activating the exposure control
circuitry.  Before the mirror begins to rise, a light value is calculated by
the camera.  Then, as the aperture arm rises to close the lens' aperture, the
photosensor of the camera detects the variation (reduction) in the light
received.  At the moment when the sensor receives the amount of light
pre-determined by the camera's exposure control program, MG3 is energized to
lock the aperture arm at that position.  Thereafter the mirror begins to rise,
and when it nears it's full up position, the first curtain is released,
beginning the exposure of the film.

(If the light value initially received by the sensor is already below the
threashold deemed necessary by the camera's program, MG3 is energized
immediately, locking the aperture arm with the lens at "full open".  If the
light value received by the sensor never reaches the level desired by the
camera's program, the aperture arm will close the lens' aperture to the opening
set on the lens, whatever that may be.  In either case, at that point the
TTL/OTF system will vary the exposure duration to control the exposure within
it's limits.)

As the first curtain begins to move, the trigger switch is operated and
exposure control reverts to the TTL/OTF system identical to that of the OM-2
and OM-2n.  Light reaching the first curtain and/or film is reflected into the
sensor, and once the sensor has accumulated the amount of light deemed
appropriate, MG2 is de-energized, releasing the second curtain, ending the
exposure.

Incorporated within the aperture arm/mirror lifting mechanism is a combined
escapement and inertia retard mechanism to slow and delay the aperture arm's
motion by approximately 12 ms to allow time for the camera to judge the closing
of the aperture, then lock it at the proper opening.

This system presupposes that the lens' aperture functions properly.  The
apertures of some non-Olympus lenses do not close properly (i.e., they may
continue to close even after the aperture arm is locked), or they may
"bounce".  The TTL/OTF exposure control will then do it's best to compensate by
varying the exposure duration, but there may still be variations or conditions
that exceed the cameras ability to deliver a proper exposure.

The apertures of some lenses (Olympus and others) are so large that they cannot
operate quickly enough for the Program function.  Among Olympus lenses, these
are the 180mm f/2.8, the 250mm f/2.0, the 350mm f/2.8, the 600mm f/6.5 and the
1000mm f/11.  These should be used in "Auto" or "Manual" modes only.

The aperture of the Olympus 35-105mm zoom may also continue closing after being
locked, but the TTL/OTF system should compensate and provide proper exposures.

(There are several other unusual lenses with a variety of "considerations"
unique to their design -- the Shift lenses, for example, require preset
apertures.  I won't bother listing these.)

Like the OM-4 and other later-model OM's, the system that displays the exposure
reading (and in the OM-2s and OM-PC determines the aperture in "Program" mode)
is controlled by light passing through a semi-silvered mirror, then reflected
off a "sub-mirror" before reaching the sensor.  Linear polarizers may therefore
not allow the camera's program mode or meter display to function properly.
Only circular polarizers should be used.

The function of the OM-PC is virtually identical to the OM-2s, btw.


Fast Primes wrote:

> Sorry if this is a dumb question, but how exactly, does the program mode on
> the OM2S work? I can puzzle out the automatic and manual modes!
>
> fast_primes
>


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