Quote Originally Posted by JohnArs View Post
It could be the back of a 100mm APO Sironar which has an diameter of only 31.5mm outside!

Cheers Armin
Hi Armin, it's ~31mm!! So lets say if it is the 100mm rear elements, what does it mean? Is the lens still usable somehow with focal length is somewhere in between 100 to 150, or is it not usable at all?