I don't recall when, where or why I acquired this Spiralite Custom Proxivar. I'm sure the accessory close-up lens did not fit on anything I had at the time. I came across it in a box recently and realized the Proxivar would screw onto my Nikkor lenses.
The instruction sheet in the box provides a good description of the capabilities:
The SPIRALITE CUSTOM PROXIVAR is a most remarkable auxiliary close-up lens which provides a continuous focusing range from about 30" to under 4" simply by rotation of its calibrated focusing mount.
The Custom Proxivar performs the function of the entire series of close-up lenses ranging from +2 diopters to +8 diopters while still maintaining any existing auto diaphragm feture and without affecting exposure.
I had half a roll of Kentmere 400 left in my Nikon F, so I screwed the Proxivar onto the Nikkor 1.8/50mm and shot some pictures around the house.
The Proxivar's magnification adjustment is very smooth and the pictures it makes show no diminution in the quality which is expected of the Nikkors. At small apertures there is a tiny bit of vignetting apparent in the corners. For close-ups with maximum depth of focus a tripod is essential, but it is feasible and fun to explore the selective focus capability provided by the Proxivar with the camera hand-held.
Spira was also a collector of photographic gear. His vast assemblage of over 20,000 items became the basis for his book, The History of Photography As Seen Through The Spira Collection. I was pleased to find used examples of the book online and am looking forward to reading it.