At 06:11 17/01/99 -0800, you wrote:
>I just tried setting my OM-4T to ISO 3200 and setting the lens to f/2.8.
>
>With the lens covered, I get a reading of about 1/4 s.
So that's about 2 or 3 stops better than mine.
>I assume the OM-4T uses a photo diode for its TTL (ie, off the focusing
>screen) readings. Any photo diode will have a very weak output, even in
>"zero" light. * This is (probably) why you get a reading even when the
>lens is capped.
>
>The question this raises is just how the camera can provide accurate
>(ingnoring reciprocity failure) four-MINUTE exposures in OTF mode. Does
>the OTF mode use a much-higher-quality photo diode? Or is there
>something else about the design of the camera that negates the problem
>of the dark current?
AFAIK the same sensors are used for both OTF megtering and the viewfinder
indication. I don't think the camera's capable of a four *minute* auto
exposure at 3200 ASA, that refers to the 25 ASA range I think. My 2SP will
happily stay open for a couple of minutes when set to 25 ASA, but when set
to 3200 the max time is about 3 seconds.
The problem appears to be with the meter bargraph rather than the OTF
control. There's obviously a leakage current or a minimum EV value that
the meter can *indicate*, and which is different to that which the OTF
system uses to determine auto exposure.
This is another point in favour of the OM-4 over the OM-2SP - because the
meter cannot indicate exposures of more than 1 sec, you can't use the 2SP
in spot mode in low light conditions (i.e. >1 sec at max aperture). The 4
and 4T can use spot metering in auto mode too so you have the usual couple
of minutes maximum. Once, when shooting with Kodachrome 25, the light
level was too low for my Minolta Spotmeter (which is nowhere near as
sensitive as an OM's meter, it bottoms out about EV1 as opposed to EV-5).
I was using my Mamiya 6 alongside the OM-4, and couldn't get a light
reading for it. So I took the shot with the OM, counted the seconds the
shutter was open, and used that "reading" for the Mamiya. Worked fine :-)
Richard
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