Hello everyone. I am looking to acquire a Nikkor 135mm for a trip, and I've found myself with several options. Consensus has it that the 135 2.8 AIs is an extremely high quality piece of equipment. But, I want to know if there is a serious optical difference between the NAI, AI and AIs versions of this lens. If it is worth my while, I will buy a NAI version and send it off for conversion, as long as it is optically comparable to the newer versions of the lens at the same 2.8 f stop. I hope someone can summarize the differences between the generations and offer an opinion whether the extra money is worth spending, or if a negligible difference exists. I thank you kindly. -R