The focus ring, the ring on SLR (single lens reflex) lenses that allows the lens to be manually or automatically focused, is a moving part lubricated with damping grease. This grease will sometimes dry when the lens has been unused for a great length of time, causing the focus ring to feel stiff. Some Nikon lenses are made with a fluted-metal focus ring to intentionally be stiff, but these and other lenses can sometimes feel too stiff for easy or comfortable use. While the damping grease should be removed and replaced when it dries, photographers may also use a home remedy to re-moisturize the grease and loosen the focus ring on a Nikon lens.
Rotate the focus ring clockwise and counterclockwise a few times. Focus rings can sometimes feel stiff if they have not been used in a while. Manipulating the focus ring can often be enough to loosen it.
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Warm the Nikon lens by cupping the focus ring in your hands. Cold temperatures cause the metal parts of the lens to contract, which can result in the moving parts becoming stiff. The heat from your hands should be enough to warm the metal and cause it to expand, loosening the focus ring.
Apply a dot of gun oil to the focus ring every 90 degrees around the circumference of the Nikon lens (at the very top, right, bottom and left side of the focus ring). Rotate the focus ring fully clockwise and fully counterclockwise a few times. The gun oil will rehydrate the damping grease, but it may take a couple of hours for the oil to be fully absorbed.
Reapply a dot of gun oil every 90 degrees around the focus ring if the ring is still stiff after three hours. Rotate the focus ring a few times, and let the lens sit overnight.
Enlist the aid of a professional Nikon repairman if the focus ring is still stiff the next morning. It is likely that the damping grease will have to be cleaned off and reapplied. This requires the lens to be completely disassembled. You should not attempt to disassemble the lens yourself unless you are experienced, as lenses contain a number of complex and very precise parts that will cease functioning if not reassembled properly.