I recently purchased a Nikon 20mm ais at a swap meet. In the first few weeks of use, I've noticed that the lens is a bit "wobbly", i.e. the part of the lens with the focus ring is a little loose within the rest of the lens (wish I'd noticed this at the time of purchase, but given the context I didn't try out the lens as thoroughly as I might have otherwise). It is noticeable enough that when I wobble it I can see the image in the viewfinder wobble a bit as well. As far as I can tell it isn't affecting the image quality. I'm wondering whether this is a common problem, should I bother repairing it, and how expensive the repair might be. Thanks for any info.