Welcome to the list!
The OM-2n has 2 completely separate exposure systems in auto mode.
What you see in the viewfinder is based on photocells reading the light
level of the image on the focusing screen.
The Auto exposure system reads the light reflected off the first shutter
curtain and the film itself. If you look in at the curtain when the
shutter is cocked, you will see a randomish pattern of black and silvery
squares that approximate the average reflectivity of film. There is a
photocell in the bottom of the mirror box that points back at the film
to measure the light Through The Lens (TTL) in real time. The Auto TTL
system doesn't work properly without film in the camera. It sees the
black pressure plate instead of film and gives longer exposures than it
would with film. When new, they came with a light gray piece of light
cardboard in the film plane so they would 'work correctly' in the shop.
There are several reasons why the 2 systems may not give the same exposure:
-Either or both of them may be out of adjustment
-The light may change during the exposure. The viewfinder system is
just an estimate; the TTL system will react to changes in light during
exposure.
-The viewfinder estimates the exposure based on light coming through
the lens when it is wide open. Any difference from the estimate in the
actual light coming through the stopped down lens will change the exposure.
-The viewfinder system is subject to being affected by bright light
coming in through the viewing eyepiece which will not affect the TTL system.
-The viewfinder reading has to be interpolated by eye. The TTL system
has hundreds or thousands of steps between stops the marked speeds.
- Others that don't come to mind at the moment, but I hope you get the
point.
How can the 2 disagree by 1-1.5 seconds when the viewfinder system
doesn't read below 1 sec.? Assuming you are projecting longer exposures
from shorter ones at different aperture/flim speed combinations, how far
off are they, 1 sec. out of 2, out of 30, out of 120? Remember 15 sec
out of 30 is one stop, 7.5 is 1/2 stop, 3.75 is 1/4 stop and anything
less than that is generally below the combined variability is the
various mechanical and chemical components of exposure, including
variability from roll to roll of film itself. In the practical world,
film is subject to reciprocity failure, a fancy term meaning the its
sensitivity to light becomes non-linear with time at low light levels.
For example, it will take more total light energy spread over 2 min. to
reach the same density in the developed film as the amount required at
1/2 sec. This varies considerably with different films. Thus long
exposure metering is not reliable, no matter how accurate the meter,
unless one knows the reciprocity characteristics of the partucular film
being used. Those who do a lot of long exposure work know about how much
additional exposure is needed and just use the exposure compensation
dial. Bracketing on the apparent over-exposure side is prudent with long
exposures.
Properly adjusted, the 2n had an excellent low light exposure system.
Do not use alkaline or lithium batteries in any of the OM bodies except
in an emergency. There are long and often complex discussions about why,
but the point is that they are not electronically suited to this use,
will not give reliable performance and will fail prematurely. The 357
and SR-44W are the best choices, but any silver oxide battery in the
correct size will out perform any other chemical system.
Moose
ajai krishnan wrote:
Hi,
I have just bought a old OM2n with few lenses. I was a bit
confused with the way the meter works. Or rather the way exposure
takes place. In the auto mode, in low light condition, the
viewfinder information does not match with the actual exposure.
i.e, the exposure time (shutterspeed) is much more than the
shutter speed shown in the viewfinder. This I have been able to
notice only for the low light condition. I posted this question in
the 'photo.net' and have got a very helpful reply. That is, I have
been experimenting this because there was no film in the camera. I
believe there is something called OT-F which will calculate the
exposure overriding the viewfinder information. Please any of you
could throw some light on this subject. I tried the same
experiment with the film inserted and now it is giving more or
less the same exposure as per the viewfinder. But still about ! 1
or 1.5 seconds difference in very low light situation. I need
another information - is it ok to use LR44 (alkaline)battery for
OM2n as Iam unable to get SR44 (silver oxide) batteries. With
thanks. Ajai
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