What is the Aperture Control Unit?
 
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.
 
How do I know if the aperture is not working correctly? 
 
An indication that the aperture unit is not functioning correctly is the exposure of the image. The images will have a dark tinge to them or when you look through the view finder it also has a dark tinge. 
 
Can the aperture be repaired?
To rectify this fault a new aperture control unit will be required.