Nikon 20mm f4 Nikkor AI - Lens

Nikon introduced their first SLR, the Nikon F, in 1959. Alongside it they unveiled a series of lenses and a new lens mount that would carry them to professional success for the next decades. To this day, all Nikon DSLRs use the Nikon F mount.

In the beginning, Nikon lenses communicated aperture information to the body using a metal coupling fork, affectionately known as "bunny ears". Soon after, Nikon began using a similar system that automatically told the camera the maximum aperture of the lens. This Automatic Indexing (AI) system stuck around for decades in lenses and is still found on Nikon's high-end professional DSLRs even though new lenses communicate this information electronically.

With only minor compatibility issues (don't use a pre-AI lens on AI bodies or modern digital cameras), Nikon F lenses and bodies are pretty much interchangeable.