I have been unsuccessful in finding specifications Nikon AI aperture rings. I recently did perform such a conversion for a 50mm H lens that I got on the cheap and just "eyeballed" the location of the a tab relative to the aperture setting and it seems to work fine. I would like to get more accurate angles relative to the indexing "rabbit ears" if possible so I can nail it exactly with my rotary table. If anyone has a link to dimensional specs for AI rings, that would be greatly appreciated. And in a related question, does anyone know why the AI tabs seem to vary depending on the widest aperture of the lens, whereas the rabbit ears all seem to be located at F5.6?