[Brightcolours]

http://www.chr-breitkopf.de/photo/aiconv.en.html

http://pindelski.org/Photography/2013/02...sions-diy/

http://rick_oleson.tripod.com/index-107.html

https://nikonmflens.org/index.php/discus...ai-nikkors

http://richardhaw.com/2016/01/21/diy-ai-conversion/

 

Also have a look at a 55mm f1.2 Ai (and 50mm f1.2 Ai/Ai-S) which shows the starting point on the left very clearly (to get correct indexing):





 

So indeed kinda at the f5.6 2/3rds stop.

 

The aperture prong part should be nonsense. There were exactly 2 Nikon film DSLRs which allowed you to see the set f-value on the aperture ring, I seem to remember, and for those there was that 2nd row of f-value numbers, and the holes in the prong to look through. Not important as far as I know. The prong used to tell the camera the set aperture on the ring. But with your DSLR that mechanism is not there anymore. You can put it on the lens to have it keep its correct Nikkor MF look. And to keep the two screw holes filled.

 

One nice aspect of this 55mm f1.2 lens is that Nikon made a special series of it (about 9 in total if I remember correctly?) especially for NASA which were used in the Apollo moon missions (so not just Hasselblad cameras), and it is said that a few of those were left behind on the moon? If it was good enough for NASA, it is good enough for our DSLRs, right?

 

Another useless tidbit about Nikkors: You can tell Ai and Ai-S lenses apart by just looking at the aperture scales. Ai-S lenses have the smallest aperture number in orange. Does it matter whether a lens is Ai or Ai-S? No. But sometimes Ai and Ai-S versions have different optics, they do have a different mechanical design sometimes and the number of aperture blades may be different. So just handy to know to identify a certain lens.