Now, the _real_ question behind this is: how the heck can the camera know the exact position of any one particular f/stop?
I think this has been discussed here on the list before, but it's got to be at least two or three years ago, and I can't recall the conclusion...
This is an easy one, at least in the case of the lens being set to a non-A aperture.
The camera doesn't need to know the exact aperture that the lens is set at. All it needs to know is how many stops down from wide open the lens aperture is set to. It meters the amount of light falling on the sensors with the lens wide open. So if it meters 4 times the amount of light falling on the sensor for a correct exposure wide open, and the aperture ring is set for 2 stops closed from wide open, it knows that the exposure is correct.
The adapter for screw-mount lenses pushed the aperture coupling ring all the way over to its wide-open position, so the camera meters the light falling on the sensors as is. Thus you get correct stop-down metering with the screw-mount lenses as well.
For simple weighted-average metering, this is all you need.
-Mat
