The Lens Mount Cap for Pentax Q-mount Lenses by Pentax is a spare or replacement mount cover (rear lens cap) for any and all of the Pentax Q-mount lenses. Q-mount lenses are specifically compatible with the Q-series of compact Interchangeable Lens Cameras. The lens cap is placed over the mount of the lens when it is not attached to the camera and protects it from scratches, dust, moisture, fingerprints, anything that can damage a lens while it is not on the camera.