I am not sure I can help as I have never owned a Nikon. However, I would expect if you choose A (Aperture Priority) or M (Manual) that this should set a fixed aperture. This will not normally change the aperture immediately. I think most cameras remain at full aperture until you fire the shutter then immediately stop down to the aperture setting for the exposure. If this is not happening there may be a fault but perhaps one of our Nikon owners can comment.