What is non-AI Nikkor Lens?

What is non-AI Nikkor Lens?

Well first & foremost AI stands for Automatic Indexing. It is a feature that allows for metering with the lens wide open, so you can see to focus with plenty of light. When you press the shutter, the aperture snaps down to that selected for the exposure.

Non-AI were early bayonet mount lenses that did not have this indexing feature. They were the first generation of the Nikon F mount lenses introduced in 1959.

Important: These lenses can't be mounted on many of the later bodies without causing damage because the "prongs" strike the body prism when you mount them.

It is possible to convert a non-AI lens to an AI lens. This is accomplished by replacing the aperture ring and the metering prong or by modifying the original part.

Deep Blue 2 gave you a great answer I just giving a link to see the difference.

http://www.nikonlinks.com/unklbil/nomenclature.htm

Deep blue 2 is correct but I'll add that non AI lenses will also damage the AI lens mount on all but a few AI cameras. Many non-AI lenses can be converted to at least partial AI function, Nikon sold AI aperture rings for some non-AI lenses, ten minutes and a screwdriver to make the change, and some rings are still available--I just this year converted a very nice non-AI 50mm/1.4 with a factory ring. For some lenses, the rings were never available or the supply of conversion rings is long gone, so third party guys have stepped in to convert these lenses.