Olympus-OM
[Top] [All Lists]

[OM] OM Specs

Subject: [OM] OM Specs
From: HI100@xxxxxxx
Date: Mon, 22 Oct 2001 23:59:22 EDT
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<<

< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >


<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz