After reading the recent posts on the OM-2s vs. the OM-2n, I dug through my
files for a copy of an old Pop Photo report on the OM-2s and found a detailed
description of how the aperture automation works. I've posted an excerpt
below. So, all ya OM-2S lovers, here's what makes that sexy shutter sound:
"F-stop automation is not so simple. The OM-2S does this by monitoring the
light coming through the lens as it stops down toward a predetermined
opening. When the photocell detects the desired amount of light, an
electromagnet prevents the lens from stopping down farther.
In practice, this method suffers from the accumulation of tolerances,
backlash, and inertia exhibited by any complex mechanism. The result is an
inconsistent over- or undershooting of the aim-point for the diaphragm
opening. To compensate, all such systems continue to monitor the light coming
through the lens until the diaphragm has come to rest. Then a final
(stopped-down) reading is made, and compared with the nominal aim-point. Any
under- or overshoot diaphragm is compensated by making a last-minute "trim"
of the shutter speed. The result should be correct exposure.
Here's the sequence as it occurs in the OM-2S, involving two geartrains,
three electromagnets, and real-time monitoring of light coming through the
lens. (The latter is accomplished by the same mirror/photocell optical system
first used in the OM-4; see "Lab Report," July '84.)
Pressing the camera's trip button fires a first electromagnet that releases a
timing geartrain. Its initial motion releases the diaphragm actuator,
permitting the lens to begin stopping down. As it moves, the actuator pushes
against a second geartrain. When the photocell detects the amount of light
called for by the exposure circuitry, a second electromagnet is fired. This
stops the second geartrain, and thus the diaphragm dead in its tracks.
Meanwhile, the timing geartrain has continued to move. Thanks to its inertial
governor, it moves slowly enough so that its full motion provides plenty of
time for the lens to stop down completely, if required. At the end of the
timing geartrain's motion, it releases the mirror.
At the top of its swing, the mirror releases the shutter's opening curtain,
whose dappled black-and-white pattern simulates the average reflectance of
film. All this time, the photocell has been monitoring the light coming
through the lens, first through the special reflex-mirror system, then
directly off the film plane. The monitoring continues until the circuitry
terminates the exposure by releasing the closing curtain with a third
electromagnet. This kind of exposure control may be most useful in scenes
calling for long exposures, during which the light level changes suddenly."
While I was at it, I managed to scan in the entire 'stripdown report' from
what the above is excerpted from. If anyone wants a copy, please e-mail me
privately.
Bill Rice
Round Lake, Illinois
brice1021@xxxxxxx
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