Hello, all.
I'm new to the forum and just getting my feet wet with vintage Nikon gear. I recently purchased an old case containing a Nikon F Photomic that contained a couple of lenses, including the tube in the attached image. I'm wondering if anyone here might know what this is for? As you can see in the images, it has a tiny lens at one end and standard F mount on the other. The lens is threaded around the outside (perhaps for an EL-Nikkor?) and behind that there is a ring that can be turned to adjust for distance. There is a little cut out in the ring allowing markings in feet to show through. The etching around that cut out says, "INF TO ____ FT." A fixed arrow just behind the ring indicates corresponding f/stops.
Thanks in advance for any light you can shed on this mystery lens!
Best,
Jeremy