John H wrote:
>>
Exposure times in the 2 are adjusted with the led in use, so times are
accurate.
<<
Yes and NO. This is true only for shorter exposures well away from LED limit
time, for the particular ASA set.
The issue here is that the LED in the OM2 adds a baseline very low light
level to all exposures. So even if the autoexposure calibration takes place
at two light levels (two point cal) and includes the LED error, it will be
off (a small amount) at all other levels since the exposure is made with the
sum of ambient light off the film and the LED light. The problem is the
baseline LED light adds addtitional curvature to the calibration which cannot
be totally removed by a two point calibration. In practise, as commented in
the original post, it is only at very long exposures (10's of seconds) that
this has any practical effect at all, since there are much bigger errors
caused by mechanical and other sources of irreproducibility, and calibration
takes place at much shorter times where the added curvature is negligibly
small..
One can do a thought experiment to see how it all works: Consider an
autoexposure in complete darkness, limited to say 2minutes by the LED, and
then compare this to an accurate (no LED present) autoexposure which should
have taken 2 minutes. Now with the LED present this exposure will go to about
1minute since the light from the film and the light from the LED are equal.
i.e. each on it's own would have given 2 minutes exposure. (error= -1 stop)
If you calibrated this error out at 2minutes it would then give about minus
one stop error at all normal short exposures. In reality this is all academic
unless one really expects the exposures to be accurate as you approach times
near the complete darkness LED limit time. And you would definitely not want
to try to perform a calibration near the LED limit time where the added LED
curvature is large! The OM2N avoids this all by not adding any baseline LED
light until the time gets out near the limit time. The reason for this of
course, was not so much an accuracy one but to reduce ASA dependence of limit
time and avoiding having a battery killing mode.
Regards,
Tim Hughes
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