I have read dozens of explanations of the difference between the AI lenses and the AIS, and I'm still not sure I understand all of them. The reference books are either not clear, or inconsistent. Here's what I think the AIS lenses have which the AI lenses do not. Someone let me know if this is wrong. The "ear," if it has one, has holes on either side of the index slot to admit light to assist in viewing the "little" f-stop marks on the aperture ring from the viewfinder. Those little f-stop marks are orange (maybe). There is a little "scoop" on the back face of the lens which engages a pin on the camera mount (maybe). Although I have read that this scoop transmits focal length information to the camera, I believe in fact is only transmits a signal that the motion of the stop-down lever on that AIS lens is linear. If it is linear, and the camera is so equipped, it will not adjust the shutter speed to compensate for slight variances in the actual closed-down aperture. Otherwise (on AI lenses) the shutter speed will be adjusted to compensate for variances in true aperture compared to nominal aperture, just after the lens stops down during the actual exposure, but just before the shutter is fired, as the motion of the stop-down lever on the AI lenses is not necessarily linear. The only Nikon camera I am sure utilizes this feature is the FA, although the N2000 may also. I had thought my FG did, but I found no sensor on the lens mount to detect the presence of the "scoop," so I don't see how it can. What I am not sure about is when Nikon added the little tab which is on the back of the lens mount, in the same radius as the stop down lever, but maybe 30-45 degrees offset from it, which is sort of molded in on later lenses. I thought it was first added to AIS lenses, but when I checked all my lenses, I realized all my AI's are only AI'd, so I can't tell. I believe it is this "tab" which communicates the focal length of the lens to the camera, so it knows whether to use a higher shutter speed with longer lenses in "Program mode." The "tab" is offset by different amounts for different lenses according to focal length, I think. My FG does have a sensor for this tab. I know these things are communicated electrically by the AF lenses. That's not my question.
