The ASA setting provides both an electronic and a mechanical input to the
system -- the mechanical input affects both Auto and Manual modes, the
electronic only Auto.
A cam on the ASA mechanism mechanically sets the meter movement so that it
is properly positioned for both Auto and Manual settings. Another
mechanical input also positions the meter according to the aperture set on
the lens.
When the mode selector switch is set for Auto, the meter circuit is
calibrated with fixed resistors so that the needle points to the approximate
shutter speed the camera may deliver (varying according to the light that
strikes the two CdS cells by the eyepiece), with the meter mechanically
positioned by the ASA and aperture inputs as noted above. In Manual mode,
the shutter speed selected is also input into the equation via a stepped
resistor connected to the shutter speed dial.
The mode selector switch has a dual function -- the circuit on top largely
affects only the meter, but is linked to a mechanism that sets the Main
circuit (below the mirror) for either Auto (variable) exposure or Manual
(fixed) speeds. It is the switch on the Main circuit that selects either
the Auto exposure function, with it's comparator/amplifier circuit (affected
by the ASA's VR input) to vary time according to the light reflected into
the SbC sensor, or the fixed resistances input via the shutter speed dial.
>
> BTW, after trying to understand circuit diagrams of the OM-2 still can't
> figure out how is it that ASA dial (AR circuit board) gets disconnected on
> Manual mode. Seems this part of the circuit is 'on' only on Auto mode,
then:
> how does CdS meter gets its info on which ASA setting it's working on
while
> on Manual mode ?
>
>
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|