I had an issue where the cooler that holds the foot rotated on my 600TC when I was carrying the camera and lens on a monopod. I was contacted by a person on BCG who mentioned that he had the same problem when he removed the hood and held on to the foot. I sent the lens in a few days ago. Nikon said it was a stripped socket and repaired it in a day. They're returning it to me today. I just wanted to let you all know of the potential issue and easy fix. No cost. They're also cleaning and inspecting the lens. A three-fer.
Marty
Nikon's latest report said it wasn't stripped, but they replaced the bolt holding the collar.
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