Quote Originally Posted by Les Sarile View Post
I always assumed that the sequence of events for all aperture priority capable manual bodies start with the lens being stopped down followed by a last meter reading at actual aperture setting and that is the case with the FG. Like you, I was able to mount a pre AI lens on my FG that doesn't interfere with the tab and in aperture priority mode tested and verified the assumption. Set it wide open and it fires at the metered speed. Turn the aperture to smallest and as expected the meter is the same as indicated wide open as the tab is not moving. Fire the shutter and now the exposure time is much longer as appropriate to the small aperture.

Of course aperture priority capable manual bodies that meter off the film - like the Olympus OM2, 2S & 4 as well as the Pentax LX, will continue monitoring the scene in real time for light changes and varies exposure time accordingly.
This is not the sequence of events for all aperture priority bodies. Imagine the shutter lag this would add to camera that use a mirror relay meter! Also, the whole point of Nikon's AI-S 'upgrade' with respect to aperture linearity would be lost. What you are experiencing is due to the FG having some elements of the FA's 'closed loop' exposure system- if it detects that the lens isn't AI-S, it tries to correct the exposure.