In 1981 Nikon introduced their AIS (AUTO INDEXING SHUTTER) lenses even though no Nikon could use AIS features until the FA in 1983. AIS lenses look a lot like AI lenses, but have their smallest F/stop marked in ORANGE. AIS lenses also have: 1) a little indentation in the stainless steel lens mount to indicate that a lens with a linear action diaphragm was mounted.