Originally posted by UncleVanya The distinction is lost on me.
Not a problem. All open-aperture meter cameras are auto-aperture, but not all auto-aperture cameras feature open-aperture metering.
I shot for 12 years with a stop-down meter camera (e.g. Spotmatic). Without automatic aperture, it would have been a pain. With auto-aperture, it was easy. Use the meter switch to stop down the aperture and turn on the meter and adjust shutter/aperture to center the needle. Turn the switch off (aperture opens fully), focus and shoot as much as you wish until light or subject changes. Auto-aperture takes care of opening/closing the iris for each shot by flipping a "paddle" in the mirror box that engages the stop-down pin that protrudes from the back of the lens.
Open aperture metering, OTOH, allows metering with the lens wide open. This is accomplished by coupling the aperture ring to the body's meter mechanism. That coupling is the non-crippled part of the non-crippled K-mount and one of the distinguishing features of the S-M-C and SMC Takumar lenses. The Spotmatic F features manual open-aperture metering allowing the meter to remain active while actively shooting and for the user to adjust exposure if needed. As with the regular Spotmatic, aperture automation is handled by engaging the stop-down pin on the lens rear. Operation is similar to the KX, KM, and K1000.
Herein is where some of the confusion arises. S-M-C and SMC lenses have a second, very small pin on the lens mount face that has nothing to do with aperture automation, but does help support trouble-free open aperture metering. That pin allows the lens to detect when it is mated to an open-aperture meter body and engages a lock on the A/M switch. This sort of interlock prevents having the lens stopped down when doing open-aperture metering (the aperture ring coupling still being engaged).
As for the OP's ES II, it and the ES add aperture-priority exposure automation to the mix. In use, both models operate in a manner very similar to the K2, ME Super and numerous other Av/M capable cameras that use the the original K-mount lens.
Steve