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With an M lens on the camera, the camera thinks the lens is open as far as it can go regardless of the ring setting because there is no electrical connection to read or control the aperture. In manual mode, pressing the green button will read the amount of light coming through the lens in accordance with your aperture setting on the ring and then adjust the speed to give the correct exposure. The camera can't adjust the aperture (no connection). In any mode other than manual, the camera can't read the aperture so it defaults to wide open because it is confused. In the manual mode, the camera still can't read the aperture because there is no electrical connection so, pressing the green button alows it to read the amount of light coming through the lens. On camera without a green button, I believe pressing the AE-lock button performs the same procedure.
Since the metering mode is set by the camera and not dependent on the lens, I'm guessing that it will meter according to the setting (spot,center etc.)
This is my understanding about M lenses and the green button. Anyone disagree? If I'm wrong, I'm back to square one even though my exposures are correct when using an M lens.
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