You _do_ need to AI-convert the lens for the prosumer FM2, FE2, and FM3a as the body's aperture follower-tab does not flip down on those cameras. If you try to mount a non-AI lens, it will likely damage the plastic aperture follower-tab, and may even jam the lens on the mount. The older F3, F4, FM, and FE all had metal, flip-down follower-tabs, so can directly mount a non-AI lens (as long as you remember to flip the tap down!). The F5 and F6 did not come with flip down tabs, but Nikon reportedly offered a modification to install a one, though I personally have never seen such a modded body in the flesh.