Nikon 200mm f/4 AI

[kit2hubsch]

A picture of puppy shoot with XT-1 and adapted nikkor lenses. They're like 30 years old but still do a good job. There is small piece of metal coming out of aperture ring. Any one know what is it's purpose?

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[Max_Elmar]

Ai (stands for "aperture indexing" - 1977 to present) Nikkors have a small tab on the aperture ring to engage with and communicate the selected aperture to the meter. There is also a small lever that sticks out of the mount that allows the camera to operate the aperture blades. AF and AF-D lenses have them as well. Newer G type lenses omit the former and the newest E type lenses the latter. The later 200/4 is a super lens! Small, light, super cheap, and quite sharp. I still use it on my XE-2...one of the only adapted lenses I still use. 

[kit2hubsch]

Ai (stands for "aperture indexing" - 1977 to present) Nikkors have a small tab on the aperture ring to engage with and communicate the selected aperture to the meter. There is also a small lever that sticks out of the mount that allows the camera to operate the aperture blades. AF and AF-D lenses have them as well. Newer G type lenses omit the former and the newest E type lenses the latter. The later 200/4 is a super lens! Small, light, super cheap, and quite sharp. I still use it on my XE-2...one of the only adapted lenses I still use. 

tx for your explanation