http://www.nikonchat.com/nikkor-ai-lenses/
What is the automatic indexing (AI) on Nikkor lenses and how it works?
Im going back 20 years on here, but I’m curious. AI and AI-S lenses Nikon appears – What does that mean? PS now the proud owner of a FM2N. Yay!
automatic indexing, or AI, allows the camera meter to measure exposure through the lens at its maximum aperture. (So you can focus and compose with a bright viewfinder) Then when you release the shutter, the camera stops the lens down to the selected aperture. The “s” in AI-s allowed for the Program AE exposure among others [harder] uses – mostly with the linearity of openness. Beyond that, there is no difference between a good or the goal of AI AI-s. If you are using a camera without the program mode, like a Nikon FE2 (and FM2n), there is no difference – but it must be an objective of the IA. (Otherwise, you must cease to wear down measurement)
Cliff Mautner talks about Nikkor Lenses
Nikon News