on 25/05/2004 17:17, Carlos J. Santisteban Salinas at cjss@xxxxxxxxxxxxx
wrote:
Hi Carlos,
> OK, but not only the distance is critical -- LED orientation would also
> matter *a lot*.
I understand. IIRC, the setup considered LED orientation.
The LED was the power-on indicator of an amplifier mounted on a rack. I took
painstaking measuring so as optical axis was at the LED's height, and camera
body parallel to the amplifier faceplate. Focusing at minimum distance was a
safe measure to guarantee constant LED to film distance. Now that I'm
remembering better what I did, I doubt if distance was set at 1 meter for
this test, and 0.45 m for the OM2 ASA/exposure test. Exposure results seem
to indicate this second test was done with the camera closer to the diode.
>
>
> 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>
Good to know, thanks for the link.
>
> 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.
Understood.
>
> 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.
This OM2n P-circuit has no IRED, if I'm following your explanation.
>
>> 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).
So it's internal LEDs vs. P-circuit.
>
> 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 ;-)
Well that's why I bought it... the C*non A1 hardly reaches 30 sec. and that
was considered *long* when came into production ;))
>
> Be warned that, at low ASA settings, max. exposure time will be *really long*.
But not overexposed IMO. Maybe that's why I rarely compensate beyond +1.
I've just checked a 1991 E100 picture of Notre Dame viewing its rear façade
during a cloudy night, which I remember to have compensated, not because of
color shift due to reciprocity failure but to gain detail in the shadows. I
think it came out acceptable, with details on the cloudy sky and on the
artificially illuminated trees of L'île de la Cité, while Notre Dame came
out brown monochrome with fine architectural details.
G.Zuiko @ f2.8, maybe some 2 minutes exposure, little tripod on the border
of Pont Neuf (was that it's name?) - I risked my camera that night !!
>
Good Venus transit, Carlos. It won't be seen as good from here.
/F.
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