okay
John Hermanson
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----- Original Message -----
From: <HI100@xxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Monday, October 22, 2001 11:59 PM
Subject: [OM] OM Specs
| 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
| >>Hi100@xxxxxxx<<
|
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