Nikon aperture ring specifications for AI conversion?

I have been unsuccessful in finding specifications for 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 yoke "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?

You won't find a spec for the position of the meter coupling ridge relative to the rabbit's ears because it is positioned relative to the maximum f-stop not relative to the absolute f-stop. The rabbit's ear system originated with an early Nikon F metering head which was not TTL and this needed absolute f-stop. When the TTL meter arrived relative f-stop metering was needed so you had to rotate the aperture ring after fitting so that the camera learned the maximum f-stop.

 

The AI system supplies the relative f-stop directly which is what a TTL meter needs. It is only the matrix meter which needs the absolute f-stop as well since it has to know the absolute scene brightness and an AI lens supplies this by other means.

 

Anyway, the coupling ridge is almost always 4 2/3 stops round from maximum aperture but I've not found a proper spec. for it.

 

There is some more information at

 

http://www.zi.ku.dk/personal/lhhansen/photo/mcr.htm

 

You'll see that there are exceptions to the 4 2/3 rule but I've never seen an explanation.

 

I have a 50 mm f/1.8 AIS (latest compact type) which has the ridge at about 5 stops round but my theory is that it is closer to f/2 than f/1.8. Certainly the travel from f/1.8 to f/2 does not seem to make much difference to the meter reading! I have three other AIS lenses and they are all at 4 2/3.

 

I hope this helps. Perhaps others have more information.

 

Richard

This is what I use for my AI conversions..

 

<a href="http://photobucket.com" target="_blank"><img src="http://i215.photobucket.com/albums/cc112/Juanjo_Viagran/BS/DSC_3169-1.jpg" border="0" alt="Photo Sharing and Video Hosting at Photobucket"></a>

 

and that is a AI conversion I did to a 50mm f:2, perfect metering.

 

<a href="http://photobucket.com" target="_blank"><img src="http://i215.photobucket.com/albums/cc112/Juanjo_Viagran/BS/DSC_3171-1.jpg" border="0" alt="Photo Sharing and Video Hosting at Photobucket"></a>

Thank you for all the info, but now I am a bit confused in how you turn an f-stop into an angle; i.e. Theta 1? I guess I could measure some lenses and get these numbers empirically. If there is a way to convert from f-stop to degrees or radians etc., that would be great. As for my first lens conversion, I just studied another F2 lens I had and it looked like it lined up with f11 on the ring, so that seemed good enough. I checked the metering against my other lenses and it seems to register properly, I just don't like guessing like this when I know I could get much more exact.<div></div>

Top view of Ai ridge releative to lens locking indentations.<div></div>