I just got a used Nikon 300mm f/4 AF (the older, non-AF-S version), and unmounted, when set to f/4, the aperture blades are still slightly visible (you can push the aperture lever on the lens a bit to make the aperture fully wide open, but when let go, it stops down slightly). As you stop the lens down by turning the aperture ring, the blades continue to stop down as expected. There is no oil or other defect on the blades themselves that I can see, and they appear to open and close smoothly. This differs from my other lenses with an aperture ring in that when set wide open, there is no slack or play in the lever to make it more wide open.
When mounted on a camera, it appears that wide open at f/4, the exposure is correct, at f/5.6, it is about 1/3 stop over exposed, and from f/8-f/32 it is about 1 stop over exposed.
I took a series of photos at each aperture with exposure compensation set to 0, then did the series again with EC applied to get the histograms to match. When mounted, the aperture blades appear to be fully retracted (they cannot be seen), and triggering DOF Preview when set to f/4 shows no aperture movement (it remains wide open), which is as it should be. I did this test on both a D700 and D300 in aperture priority mode, with fixed ISO (auto ISO off), 0 exposure compensation, attached to a tripod, pointed at an empty wall (I first noticed the issue outside in the park). I compared to another lens (50mm f/1.4) at the same apertures shot at same wall, and all of its exposures matched the exposure from the 300mm at f/4.
Any ideas on what the problem might be, and what it would take to fix (other than using exposure compensation when stopped down)? Since the only thing mechanically that seems out of place is the somewhat loose aperture linkage lever, I assume a spring might be stretched or weak and causing the problem? None of my other lenses have this issue, so it's not the camera bodies. Thanks!
[Edit] One additional bit of info after some further checking - when unmounted, and stopped down to f/32, then slowly brought back to bigger apertures, the control lever seems to stick at the closed down position until about f/11, then it starts to move again, but this is purely in the lever mechanism itself, not due to the aperture blades. Worth trying to remove the lens mount myself to investigate?