In reply to Richard Ross's question, C.H.Ling wrote:
>> Two years ago I found my OM4Ti have the same problem, but to a less
degree, it
>> double below 1/15s around 3 times below 2s. Don't ask me about the
battery I
>> used or was there film inside. I have a perfect OM4 without any of this
problem,
>> also the two OM2n that I had also without this problem. It is certainly
the
>> problem of design or manufacturing, where some components may degrade
with time.
>> I have took my 4Ti to OM service centre here and the technician in charge
(over
>> 15 years working there) just said it was good under his test equipment
and he
>> took me another 4Ti (his own one) and I had tried it has the same problem
as
>> mine. So now I will only use the 4Ti at spot or manual mode.
I can't comment on the particular problems of this, or any other, camera,
but there are a couple of other possible reasons for discrepancy between
manual and auto that haven't yet been mentioned on this thread.
1. OTF/OTC auto exposures are metered with the lens stopped down to the
taking aperture, whereas manual and spot readings are taken wide-open. For
the latter to meter correctly requires that:
- The *difference* in brightness the selected aperture setting and
wide-open is
accurately signalled from the lens to the body, to allow for
correction
of the metered light reading by the electronics.
- The stop-down of the lens prior to exposure precisely matches this
signalled
information.
Since both the signalling and the stop-down are controlled mechanically,
there is at least the possibility of slop or inaccuracy in the lens linkage,
the body, or both (this would be less likely to affect exposures made with
the aperture set to wide-open).
OTF/OTC auto is not affected by such errors.
2. OTF/OTC auto exposures are metered with the mirror up, whereas manual and
spot readings are taken with the partially-silvered mrror in the down
position. There is hence a possibility of light leakage from the eyepiece,
although it would have to start out pretty bright to have any effect as it
is attenuated first by the focussing screen, then by the mirror and finally
by the shutter curtain.
Since (IIRC) only one of the shutter curtains carries the metering pattern
(the other is black) the meter display would vary according to whether the
shutter was cocked or not, if such light leakage were occuring.
I don't really know how likely it is for either of these possibilites to
affect the meter reading, or to that degree, but it is interesting that in
both cases it would be the OTF/OTC auto exposure that would be correct, and
not the manual/spot value.
-Brian
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