You're correct that the OM2 was the first implementation - even though Canon and Minolta applied for patents first, and that it meters in realtime OTF. However, all Olympus are limited to 120 seconds max in their aperture priority autoexposure according to the manual as well as testing I have conducted with my OM2 and OM4. Please let me know if there is another camera that can meter for as long as it takes or battery is exhausted in aperture priority autoexposure as that is my main interest and knowing of another would be great! I started with the Canon EOS and they are all limited to 30 seconds and exposure is determined at the time the shutter is depressed.