I believe I am correct regarding the difference in aperture mechanisms between AI and AI-S and in stating that the lenses both work on the D810 but the lens aperture ring must be used to control the aperture.
Quoting from
Nikon USA :
The AIs lens was created when the aperture mechanism of the AI lens was changed to allow automatic aperture control, with cameras such as the FA and N2000. This modification means that the aperture increments of the AIS lens can be controlled more precisely by the camera.
Here's how the F-mount was originally designed to work. By setting an f-stop on the aperture ring, you controlled how far the lens stopped itself down when there was no tension on the aperture lever. When you mounted the lens, the camera held the aperture lever open until you either released the shutter or pressed the DOF lever. At that point, the camera would release the lever and the lens would stop down to the preset aperture.
That works great for M and A modes. It doesn't work for P or S mode where the camera has to choose the aperture for you. Instead, the camera would choose the point the lens would stop down to. With the exception of the new E lenses, that's how things still work when you use P or S mode or select the aperture on the camera instead of using an aperture ring.
But pre-AI-s Nikon lenses have a limitation. The action of the aperture lever wasn't linear and varied from lens to lens. For example, on my 50mm f/1.8 AI, the lever moves a lot more when stopping down from f/2 to f/2.8 than from f/2.8 to f/4. Without a CPU, the camera could only guess how much to allow the lens to stop down. So AI-s lenses were designed with a linear action, as are all modern Nikkors with CPU's.
As things turned out, though, Nikon only made a few models of cameras which supported P and/or S modes with non-CPU lenses. Those are the Nikon FG, FA, N2000/F301, and N2020/F501. With those models, if the camera detects a non AI-s lens in P or S mode, it will take a last-instant mirror-up meter reading and adjust the shutter speed to try to correct for the non-linearity. AI-s also had one more "trick." It had a focal length indexing ridge, by which the camera could mechanically detect an AI-s lens of 135mm or over. This allowed cameras to choose a different program for longer lenses. The Nikon F4 supposedly uses this feature, but I don't know how.
If you "chip" an AI-s lens, you can control it using the command dial (or dials) of a modern dSLR and it works as you'd expect of an AI-p lens. But if you "chip" an AI lens, the dSLR doesn't know about the non-linearity and you end up with the wrong exposure solution in all modes.
Complicated, confusing and obtuse? You bet. That's why Nikon gave up on supporting P and S modes for non-CPU lenses, starting with the N4004/F401 in 1987.
--
Such commentary has become ubiquitous on the Internet and is widely perceived to carry no indicium of reliability and little weight. (Digital Media News v. Escape Media Group, May 2014).
