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[OM] Re: Long exposure capability of OM2n

Subject: [OM] Re: Long exposure capability of OM2n
From: "Carlos J. Santisteban Salinas" <cjss@xxxxxxxxxxxxx>
Date: Tue, 25 May 2004 22:17:25 +0200
Hi, Fernando and all.

>> unless both cameras and the LED were
>> *exactly* at the same place in all tests, Fernando's results would be
>> useless (lo siento ;-)
>They were, Carlos, rest assured ;-))
>Focused at 0.45 m.

OK, but not only the distance is critical -- LED orientation would also
matter *a lot*.

>>> My rules of thumb on compensation are for my OM2, might not be
>>>applicable to
>>> the 2n.
>>
>> I have no reason to believe they would be different, besides maximum
>> exposure time versus film speed (constant on the 2n, variable on the 2)
>
>That's why...

AFAIK, the OM-2n adds the 'P circuit' to the 'main' circuit already present
on the plain OM-2. The P circuit, besides controlling T-series flashes,
does a kind of automatic shutoff after about 3 minutes (2 minutes is the
'guaranteed' minimum time), regardless of the ASA setting. Otherwise, the
main circuit is almost the same in both cameras -- see
<http://www.zuiko.com/PAGE14.HTML>

I don't have Mark's CD at hand to check the schematics, but I think that
the LEDs (IRED really) placed beside the SBCs are responsible for exposure
limit on the plain OM-2: if they make a very low, *constant* illumination,
a higher ASA setting would finish exposure after less time -- the same
light level would be 'integrated' faster.

When doing astrophotography (at 'B'), it's recommended to take off the
batteries from the OM-2 -- some have reported film fogging from the LEDs!
Plus, you will save a lot of batteries ;-) On the OM-2n, battery
consumption should stop after the first 3-and-a-bit minutes of exposure,
due to the P-circuit.

>My red diode 1981 test showed these results:
[big snip]

Again, I think this are the aforementioned LEDs in action... If you
stop-down the lens, less light form the outside would reach the SBCs, but
the internal LEDs don't change their level, so when you change ASA to
compensate the aperture change, you would reach equal exposure from the
outside light... but internal LEDs exposure become more relevant, hence the
shorter exposure (higher *total* measured light).

But don't worry: OM-2 (& OM-2n) are still *much* better in measuring such
low light levels than *any* other SLR (and in many other respects ;-)

>>> Out of curiosity, I just tested a 2n. Body cap on, black bag over
>>>viewfinder.
>>> Shutter closed at 3m20s. Will try the other 2n when its film is finished.
>>
>> Mine did about 3 min. I've got another OM-2n (which badly needs a CLA) that
>> closes at 3m23s.
>Well when I finish the loaded Velvia 50, I will test again my OM2_without
>LEDs_ until I have to install new pair of Renata/Varta ;))

Be warned that, at low ASA settings, max. exposure time will be *really long*.

Enjoy,

...

Carlos J. Santisteban

<cjss@xxxxxxxxxxxxx>
<http://cjss.galeon.com>



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