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Re: [OM] Re: olympus-digest V2 #1893/ OM2 timing limits

Subject: Re: [OM] Re: olympus-digest V2 #1893/ OM2 timing limits
From: HI100@xxxxxxx
Date: Sat, 14 Oct 2000 20:31:25 EDT
rdk@xxxxxxxxx writes:

<< At more normal film speeds, the OM-2 and OM-2n have about the same max 
exposure  time...... >>

This has been covered adnauseum over the years check the archives for more 
detail than you ever wanted. However one more interesting aspect is that 
although the OM2 can typically achieve longer times the exposure accuracy can 
be significantly lower at some of the long times. This is because the 2N 
contains a delay timer that switches on the (much brighter) LED after 2 
minutes or therabouts while the LED in the OM2 is always on. This is also why 
the OM2 has a much much greater ASA dependency than the OM2N. That is, the 
OM2 LED light output is fixed and so the limiting time is completely 
controlled by ASA setting as the camera integrates the LED light just like 
the normally measured light.  In the OM2N only a much smaller part of the 
time is ASA dependent since most of the time is determined by the timer 
circuit.
             We can do a thought experiment to show how the error arises. If 
we had an OM2 with say a 3minute maximum time in complete darkness and an 
OM2N with a similar maximum dark time. (we could adjust ASA setting keeping 
the cameras the same to get this exact point)  If we then set the ambient 
light level at the same level as the limiter LED in the OM2 then the exposure 
of the OM2 would end at 1.5minutes while that in the OM2 would end somewhere 
between 2 and 3 minutes. Neither would be the correct value of 3 minutes but 
the OM2N would be much closer.  
       We can say in general the OM2N will be more accurate as long as the 
correct exposure time is a little less than the OM2N's maximum limit, 
somewhere in the 2-5minute range and relativeley independent of ASA.  On the 
other hand the OM2 will be much better for times longer than the OM2N's 
limit,  but that mainly only applies in the case of low ASA settings since 
it's limit can become shorter than the OM2N limit at high ASA settings. The 
crossover point will be very dependent on the sample of OM2 used as the OM2 
limit is not as well controlled as that of the OM2N, being dependent on the 
LED stability at very low drive current where self leakage and temperature 
become added variables.

       Regards,
        Tim Hughes
       >>Hi100@xxxxxxx<<

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