When using "Green Button" metering it's important to keep your eye to the viewfinder (or otherwise shield the viewfinder) to prevent stray light skewing the metering (bin there, done that!) ... alternatively, maybe the aperture is just a teensy weensy bit sluggish so it doesn't quite get around to closing down properly before the shutter fires.
You could try setting up a Manual exposure with another lens at, say, f/11 or f/16, then having established this is working with a good choice of shutter speed and ISO, simply swap the lens, set it to the same aperture and make another exposure. If the images are wildly different it's likely to be a sticky aperture or other damage in the aperture linkage.
Good luck