The OM-System was introduced by Olympus in 1972, more than a decade later than most of the, by then, well-established 35mm SLR brands. Its success must undoubtedly be attributed to Olympus’ chief designer Maitani Yoshihisa and his staff, certainly taking advantage of new technology and increased general 35mm SLR understanding. The nucleus of this system is the brilliantly conceived camera body of remarkably compact design, yet with possibly the best viewfinder in any 35mm SLR camera. The first model introduced was the all-mechanical M-1, soon renamed OM-1, with a full aperture TTL CdS exposure meter and a wide bayonet lens-mount, gradually complemented by several quite sophisticated models. The system is also associated with one of the finest ranges of optics ever made available, the OM-System Zuiko lenses, and a generous selection of accessories.
OM-2 is the automatic version of the OM-1, presented officially in 1975 at the 31st Photo Salon in Paris. An OM-2 prototype was displayed in a showcase at the 1974 Photokina in Cologne, but none of its features was announced. Externally it is nearly the same as the OM-1, but the shutter is electronic and the exposure is either automatic (aperture priority) or manual.
The exposure meter of the OM-2 is able to measure the light reflected by the film (1/45th sec and longer) and compensate for any variation of light during long exposures; this is called off-the-film (OTF) metering. The exposure sensor also controls the flash exposure; this is called through-the-lens (TTL) flash automation or otf flash exposure. The OM-2 was the first camera to have these features (a Minolta patent licensed to Olympus). The TTL flash automation greatly simplifies flash exposure, and was quickly adopted by most other SLR camera makers, while OTF metering was also adopted by some competitors, like the Pentax LX. The Olympus Quick Auto 310 flashgun was designed for the OM-2, which unfortunately is not compatible with the T series flash units introduced in 1979 together with the new OM-1n and OM-2n.
The OM-2N, based on the OM-2, has the same modifications plus:
- a direct contact inside for Recordata backs
- an exposure compensation warning flag
- full-frame averaging at all shutter speeds
- 120 second exposure limit on auto, though in practice, low light exposures will often go to 3.5 minutes (the OM-2 limit was listed as 60 seconds in the instruction book, but at asa 12 would go as long as 19 minutes while at asa 1600 would end in as little as 19 seconds)
All these models existed in chrome or black. According to this page of the OM Sales Information File.