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[OM] Learning about the OM-2n, was: OM2n

Subject: [OM] Learning about the OM-2n, was: OM2n
From: Chuck Norcutt <chucknorcutt@xxxxxxxxx>
Date: Mon, 09 Jun 2003 08:12:29 -0400
Hello, Ajai. Welcome to the club. Your confusion is not unusual and your "problem" is not likely a real problem. The OM-2 and OM-2n have two exposure systems. The indicated exposure that you see in the viewfinder is derived from a sensor inside the viewfinder. The actual exposure is controlled by a second sensor on the floor of the mirror box. The second sensor takes its reading from light reflected off the shutter curtain or from the film. The shutter curtain is covered with a speckled pattern of black and white which is designed to average out to gray and mimic the typical reflectiveness of film. Whether the shutter curtain or the surface of the film is used to control the exposure depends on the shutter speed. This second sensor is called the OTF sensor for "Off-The-Film"

When you try to operate the camera without film all the second sensor sees is the black pressure plate instead of gray film. The pure black of the pressure plate reflects very little light; the camera believes the light level to be very low and calls for an exposure much longer than previously indicated by the first sensor inside the viewfinder. To avoid this problem you can keep an old roll of outdated, unprocessed film specifically for testing the camera and use it over and over.

Be aware also that the sensor in the viewfinder can pick up stray light from the eyepiece. If the eypiece is not shielded from light by having your eye in place (and its best to use an eyecup) the viewfinder sensor will believe there is more light than that seen by the OTF sensor and call for a shorter exposure than the OTF measurement. But, if you are shooting in auto mode, the OTF sensor will still control the exposure. If you are shooting in manual mode be aware that bright light coming in behind you can get in past your eye and have an effect on the meter reading. Shield the eyepiece with your hand to get a better reading if you suspect there is a problem.

Alkaline batteries can be used but they will have a shorter life than silver oxide. The difference between alkaline and silver oxide is that the voltage drops quickly on alkaline even though is it capable of delivering an acceptable current. A silver oxide battery will maintain its voltage until its current delivering capacity is nearly exhausted. Alkalines will also have an effect on the accuracy of exposure as they are used up since accuracy is dependent on voltage. When the battery voltage drops too low (about 2 volts ???) or without batteries in the camera the mirror will lock in the up position to warn you of the condition. Press the check/reset button after inserting new batteries to unlock the mirror. My advice is look harder for silver oxide batteries. They really do make a difference.

Now, as to the overall accuracy of the camera... even with silver oxide batteries keep in mind that it is probably 20+ years old. If it has never been serviced it may need a CLA (clean, lube & adjust) and may also need its foam light seals replaced. Check the foam seals in the back for signs of crumbling (drying out) or turning to sticky goo. Check your lenses to see that the diaphragms are not sticky. The diaphragm should have a "snappy" opening and closing action with no hesitation. Run a roll of slide film (not negative film) through at various shutter speeds. If the slide film is properly exposed then the camera is working correctly.

Chuck Norcutt
Woburn, Massachusetts, USA
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 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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