AI became standard on Nikon cameras and NIKKOR lenses in 1977. Non-AI lenses coupled to the camera s meter through a system that required a pin on the camera to be mated to a slotted prong on the lens before the lens was mounted, then the aperture ring on the lens had to be turned from one extreme to the other to index the meter to the maximum aperture of the lens. AI eliminates this entire procedure because meter coupling and indexing occur automatically when the lens is mounted on the camera. Most AI lenses made until a few years ago were also supplied with the coupling prong so they would be compatible with either metering system.nikongrossary
Short for Auto Iris. Opens and closes lens aperture as light levels change, compensates for variations in light levels, from sunlight to shadows. Can prevent camera damage in very bright sunlight.
Automatic index; Nikon s system for telling the camera s exposure meter what the lens maximum aperture is.