This particular model of adapter allows you to use lenses which do not have a dedicated aperture ring. These adapters do not have any electronic connection to the camera and hence cannot "tell" the camera what aperture is set, therefore the adjustable ring on the adapter provides a means of manually adjusting the amount of light passing through the lens to the camera sensor. NB If you have lenses WITH an aperture ring you could use another simpler (and cheaper} model of adapter and manually adjust the aperture on the lens.