The OM-2 is a 35mm film slr with both manual and automatic shutter speeds (and mechanical "B").  It is an all metal body with metal top and bottom, and rubberized silk shutter curtains.   Starting at the low range, stepless, infinitely variable automatic exposure (19 min. to 1/45th second) is calculated by reading light reflected off the film in low light during the exposure.  High Speeds  1/45th through 1/1000th are set by  reading light reflected from the first shutter curtain after the mirror starts to go up.  Manual speeds are B, 1 second through 1/1000th.   Earliest OM-2 had center-weighted curtains, later production and all 2N have averaging shutter curtains.   Power is provided by (2) silver oxide 1.55v batteries (357, 303, SR-44W, G-13, MS76, KS76). 

Time exposures in very low light of up to 19 minute exposure can be had at ASA 12. OM-2 low light capability will vary with the ASA the camera is set to.  As the asa goes higher, the low light expsoure limit is proportionally shorter.  When set to asa 1600, for example, the OM-2 will have an exposure limit of approximately 20 seconds. When using high speed film, you can fool the camera into giving longer low light exposures simply by setting the ASA knob to a lower setting. 

If you are going to test low light capability of your plain OM-2, put a lens cap on and tape over the eyepiece with black electrical tape. Low light exposure with the 2N is limited to a firm 3.5 minutes (the instruction book says 2 minutes) by the P circuit found in the bottom of the camera. This circuit also forces 1/60th shutter speed during TTL flash pictures. If interested in creative work mixing ttl flash with ambient light, you would have to use a plain OM-2.
OM-2/2N Best Battery: 1.5V Silver Oxide (357, EPX76, G13, MS76, SR44, 303, S76, KS76)
No Alkaline (LR44, A76), NO Lithium (CR1/3N) 
Battery Drain Characteristics: Very rare in 2/2N unless P circuit (2N only) is defective, or camera is tampered). P Boards and main circuits are available (not for sale separately.)
As the mirror starts to move up, a switch closes and activates the
circuit built into the mirror box floor. When the diaphragm control
lever starts to move, a second switch (main switch) in parallel with
the first switch (sub switch) also closes. These two switches back
each other up in case one fails. In later production, the dual switches were replaced with a single gold plated switch. .

Automatic auto circuit "on" is one of the reasons there is no mirror lock in an Olympus SLR with auto exposure control. If you were to leave the mirror locked up, your auto circuit would always be on and the batteries would drain. 

When the exposure is complete, the mirror comes back down, the main switch opens, and the auto circuit turns "off" ready for the next exposure.
Olympus OM-2
The meter switch (next to the rewind knob) allows you to choose between "Manual" and "Auto". The "Off" position is actually a modified "auto" setting. At this position, the meter movement is turned off, but if you are somewhat educated regarding available light, you might know that at your setting of f5.6 in bright light you would get a shutter speed of 1/1000 with your asa 200 film. Without turning the meter switch to "auto", you can raise the camera to your eye, focus and shoot. You would get the proper shutter speed in this situation.
As long as there's enough available light to give you a shutter speed above 1/30th, this technique will work. If you had to turn the switch to "auto" first, you might miss the shot. 
OM-2N Tech Page
Tech Pages - OM-2/2N
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