What's the heck about Non-AI Lens, AI-Lens and AI-S Lens?

In 1977 Nikon introduced a new system for coupling the lens to the cameras exposure system. This new coupling system was called "Automatic maximum aperture indexing" or "Ai" for short. This change did not effect the traditional Nikon bayonet mount, thus allowing Ai (and the latter AF and AIS) lenses to be fitted to older non-Ai cameras.

The Ai lever on a Nikon FM2n

However, while Ai lenses where backwards compatible with non-Ai cameras, non Ai lenses where not compatible with all Ai cameras. The Nikon FM, FE, EL2, F3, F4 and Nikkormat FT3 cameras used the Ai metering system but allowed non-Ai lenses to be fitted due to the metering coupling lever being able to be disengaged. The F5 can have this mechanism fitted as an optional extra, please contact your local Nikon representive for more details. The F2A and F2AS were Ai cameras but the Ai mechanism was fitted to the removable metering prism.

A Nikkor Ai lens, note the Ai coupling ridge high-lighted by the red ring.

The AIs lens was created when the aperture mechanism of the AI lens was changed to allow automatic aperture control, with cameras such as the FA and N2000. This modification means that the aperture increments of the AIS lens can be controlled more precisely by the camera. You can tell if a lens is AIS when: The minimum aperture (both the main larger and smaller direct read out figures) are marked in orange i.e. F16, A notch has been taken off the rear bayonet mount, above the lens locking notch. This allows certain Nikon cameras to detect if an Ais lens is fitted or not. See images below. A picture of a non-Ai Nikkor lens. Note the lack of the Ai coupling ridge, however note that at the rear of the lens, the aperture ring protrudes slightly over the bayonet mount and therefore can damage the Ai lever on Ai cameras

A Nikon Ai lens note no Notch above lens locking cut out.

A Nikon Ais lens, with lens type signaling pin notch ( highlighted red circle ).