> 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. No, all AI and AIS lenses have holes on the pre-AI metering prong (rabbit ears) to help illuminate the "little" f-stop marks on the aperture ring. This scale is known as the aperture direct readout (ADR), which can be viewed directly in the viewfinder of AI cameras such as FM and FE. > Those little f-stop marks are orange (maybe). On AIS lenses the smallest aperture is marked in orange on the aperture scale *and* the ADR scale. On AI lenses the ADR scale is white, and the minimum aperture may be various colors, including orange. > 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. The scoop indicates that the lens is AIS, it does not transmit focal length information. Linear correspondance between the stop-down lever and the aperture is the most important difference between AI and AIS lenses. It allows the camera to accurately set the aperture in shutter priority and program modes. > The little tab which is on the back of the lens mount, 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. This tab is not a focal length indicator, it is a lens speed indicator. The FA, N2000 and F4 use this tab for matrix metering. Matrix meters require the absolute lens speed to work. I think these cameras and the FG series also use this for flash calculations. The lens speed tab is present on all AI and later lenses. AI'd lenses lack this tab. I think AIS and possibly AI lenses have a small depression on the lens mount which indicates whether the focal length is135mm or longer. This crude focal length indicator is used in program modes (FA and FG?) to indicate whether the program mode should bias towards higher shutter speeds or not. In AF cameras, lens speed and focal length are communicated much more accurately by electronics, especially in the case of lenses with variable aperture and focal length (zooms and macro). Other cosmetic differences between some AI and AIS lenses: The focusing scale on many AIS lenses, espcially wide-standard lenss, is more compressed. Most AI lenses take 1/2 to minimum focus, while AI lenses take only 1/4 turn. This makes AIS lenses faster to focus, but less accurate. The DOF scales are also correspondingly closer together. The DOF scales on some AI lenses is on the lens body under the focusing ring. The DOF scale on almost all AIS lenses is on the chrome ring. Series-E lenses are AIS (they were AIS before AIS existed!), except they lack the pre-AI metering prong. Hope this helps, Roland
