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Re: [OM] questions about other low ev cameras

Subject: Re: [OM] questions about other low ev cameras
From: HI100@xxxxxxx
Date: Wed, 5 Apr 2000 04:59:19 EDT
rtomcala@xxxxxxxxxxx writes :

<< Tim,
   I don't think so!  I think that you might want to read John H's post
 of yesterday or the day before.  I still stand by my statement (which
 John backs up).
 Rand E.
  >>

Rand,
          I have been considering modifying (extending)  my OM2 auto limiting 
action in "off mode" so have looked at both schematics pretty carefully.
         I have both the OM2 and 2N Olympus service manuals with full circuit 
schematics and descriptions. The OM2N service manual in particular has an 
extremly detailed comparison with timing diagrams comparing operation of the 
OM2 to the upgraded OM2N circuit. I actually posted detail about this 
previously on an old thread. (you could check archives) But will just quote 
here from the OM2N service manual : 

"AUTO maximum exposure time (auto limiter) 
In the old model ,maximum exposure time was subject to wide variation,just as 
in the case of its off limiter, with changes in ASA sensitivity setting. As 
will be noted in the following two graphs, a substantial improvement is 
secured.  
.... (detailed graphs comparing OM2 and OM2N LED timing,integrator output 
etc).....
After about 2 minutes ( 2 to 5 minutes) of the turning on of the main switch 
, the LED lights up brighter than in the old type. As a result, there is no 
possibility of the maximum exposure time stretching out from 5 minutes to up 
to 20 minutes. "

The description/specs for most of the timing limiting are really for the 
100ASA setting although this is not well spelled out in the manual and the 
limiter will have some small dependency on ASA but much reduced over the OM2.

The OM2N circuit description of the added limiter circuit also shows that you 
can trim the timing of the limiter within some restrictions (since some of 
the timing parameters interact) by tweaking the resistors. The limiter 
circuit is in the base of the camera so is relatively easy to get at to 
modify the 2N limiter action. For example you could increase the time 
modestly if your camera were on the low end of the 2-5min spec.  
      On the other hand the OM2 has a resistor that is factory selected for 
each camera to supply the LED with the right current to limit exposure times, 
this means again you could trim this to increase limiter time if you did not 
mind the possibility of exhausting your batteries with extreme length 
exposures. 

  Regards,
  >> Tim Hughes <<
  >> Hi100@xxxxxxx <<



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