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The shadow appearance may be due to the Aperture Control Unit in the camera. The aperture is a device that controls the light that enters the camera. This is the exposure control. Nikon have the aperture control unit in the body and Canon and Olympus have the aperture control unit in the lens. The camera aperture is located behind the bayonet mount. If the aperture is failing it will cause a shadow in the view finder and a dark tinge on the images.
To rectify this fault: If the aperture lever is bent we can attempt to straighten it or a new aperture will be required.
A reason the aperture may be failing/bent is if the Nikkor lens is missing the stopper screw. When you are mounting the lens to the camera this screw stops the lens from rotating to far and damaging the aperture lever
Nikkor stopper screw