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Re: [OM] OM-1n meter and ASA problems

Subject: Re: [OM] OM-1n meter and ASA problems
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Tue, 01 Jan 2002 23:45:43 +0000
I just performed an experiment with my OM-1n . . .
With the meter *off* and an *f/1.4* lens (I don't have a 50/1.8):
- film speed dial set to 80
- shutter speed ring set to "B"
- aperture set to f/1.4
Meter needle centers between the pincers.
Reason for ISO 80: f/1.4 is 1/3-stop faster than f/1.8 and ISO 80 is 1/3-stop slower than ISO 100 canceling each other out.

Henrik:
*Ensure* you have the film speed dial set to 100 for an f/1.8 50mm lens, or 80 for an f/1.4 50mm lens! With the meter *off* I can also move the meter needle around with ISO 400 and ISO 800 film speed settings using the very slow shutter speeds, or by setting shutter speed to 1 second and moving around around the wider apertures. It's an indication the meter biasing mechanism on the shutter speed ring and on the lens are working to bias the meter near the lower range of what it's able to measure (EV 2-17 for ASA 100 and 50mm f/1.4 lens).

Playing with ISO speed settings, shutter speeds and apertures, it appears as if the rock bottom for the meter (centered needle with power off) is an exposure setting of EV = 0 for ISO 80 and EV = 0.33 for ISO 100. The specs provide a 1.67 EV buffer to ensure the meter needle is off-center at EV = 2 (for ISO 100 film speed). It also appears the "B" shutter speed setting is presumed to be the equivalent of 2 seconds by the OM-1[n] metering system. This is logical. A 2 second exposure can be made by using the "B" setting and the self timer. It holds the shutter open for almost exactly 2 seconds (AFIK this is an undocumented feature).

FYI (my conclusions about OM-1[n] metering sensitivity):
The OM-1[n] specifications show a 2-17 EV metering range using ISO 100 film. The minimum luminance for its metering system can be deduced from this. ISO 100 film has a "speed value" or SV of 5. This means the luminance in "brightness value" or BV can be -3 since the sum of the two gives an EV = 2. A BV of -3 is a luminance of 0.125 foot-Lamberts [BV = log(luminance in fL)/log(2); log(.125)/log(2) = -3].
  0.125 fL is approximately:
  0.04 cd/ft^2, or
  0.43 cd/m^2
  [NO lux or foot-candles, they're *NOT* luminance units!]
This is apparently the lowest luminance level the OM-1[n] meter can reliably measure and still show under-exposure by at least a stop. It's also 1.67 stops more luminance than the approximate rock bottom for the metering.

To give a feel for this luminance level:
(a) It is approximately one stop brighter than Niagra Falls illuminated by white lights (not the colored lights which are darker) or a skyline with a distant view of lighted buildings at night. (b) It is approximately two stops dimmer than candlelit close-ups, christmas lights at night, christmas trees, floodlit buildings and subjects illuminated by street lights.

-- John

At 00:57 1/2/02, Henrik Dahl wrote:
My OM-1n has developed a strange behaviour.

- The meter activates on both "on" and "off" position. With lens on open (5.6~2.8) and in the shutter range 1s~1/60 it acts as if metering the light, but does not *react* to light. It rather sets itself to different fixed positions. In other words it doesn't matter whether I point straight into a light source or to a dark corner. - The strange thing is that it does this even with the battery taken out! So it must be some kind of mechanical fault. - With battery: in daylight, with lens stopped down and in higher shutterspeeds, it starts to react as normal to light changes. - It may have to do something with the aperature lever on the body. With lens off I can manually turn that and provoke the erratic meter behaviour.
- The ASA dial is possible to turn infinitely.
- It doesn't matter what lens I try, so the problem lies not there.

I can deal with this "bug" in daylight, but in lowlight situations it's hard to tell if the reading is accurate or if it's due to the meters "own life"
Could it be some magnetic or spring-problem? Aperature-lever? Time for a CLA?

I found this advice from John H in the archives:
------
Every OM-1 body will display this behaviour.  Remove battery, set ASA to
100, ss to B and lens to 1.8,  needle centers.
John
> >
> > RK wrote:
> > >
> > > Hi Everybody,
> > > I just noticed that my OM1's meter needle in the viewfinder responds to
> > > changes in light both with and without batteries!
> > > Is this a new way of saving on batteries or is my OM1 heading for that great
> > > camera store in the sky?
> > > RK
------
Well I tried that and sadly it didn't work. With the settings mentioned above the nedle is way up in the roof.

All this started two days ago, before that this beauty worked flawlessly. What to do?

Henrik Dahl


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