Nikon TC's have three levers. One, on the inside ring controls the lens aperture. There are two additional sliding levers on the outside of the lens that don't appear to do anything. What do they do and are they essential? The reason I ask is that I just picked up a used TC-20E an an amazing price that was rated 8/10. There are marks on the body itself, but the glass is clean. The inner lever is nice and loose, but the two levers on the OD are sort of tight. The TC appears to work fine with my D200 and 70-200VR. Should I worry about this? Is there any way to loosen them up? A little WD-40 (just kidding)?