The FM2 does not have any electrical contacts that communicate in any way with any lens. The camera doesn't know how many stops away the lens is set from the maximum aperture. It simply 'knows' how much light will enter at a given setting, combines the data from that setting with the given shutter speed and arrives at a meter reading. The aperture settings on the lens are referred to as being 'stepless', which is to say you can set the aperture ring to any setting whatsoever and the meter will respond accordingly. At the moment of exposure, the aperture will be closed down to the precise setting you made on the ring, whether it's f6.3 f6.7, etc. Shutter speeds, however, are not stepless.
Originally Posted by nwilkins