the reason it can't (or rather, doesn't) control aperture in AI lenses is because the camera comes with no way to tell the difference between AI and AI(S) lenses.
While I agree with the rest of your post, this part is not correct. Very few cameras that Nikon ever made actually made use of the lens type signal notch. I believe the N2000, N2020 and the FA were the only ones.
that's probably correct. there might be a few more, but i don't recall. i know the FA did have it, for sure, because i was looking into getting one about 10 years ago, when i was still using film.
I don't believe any of the digital bodies make use of it, which is why AIS lenses with the signal notch still won't meter on most bodies.
no, this isn't correct. AI and AIs lenses will both
meter on a D#00 and D# cameras (excluding the D100, as kitacanon points out), even though those cameras lack the signal post that detects the difference between AI and AIs. there are actually
three points of communication between the camera and an AIs lens.
1. the AI ridge on the aperture ring tells the camera how many stops the lens is stopped down from wide open.
2. the groove we've already discussed.
3. a small secondary ridge of the other side of the aperture ring that can be used to tell the camera when a lens is at minimum aperture.
this is the same little bit that tells modern cameras when an AF-d lens is set to minimum aperture.
In order for the meter to work, the camera requires two pieces of info from the lens, the maximum aperture of the lens and what aperture you've selected.
the dirty secret of AI metering is that the camera actually requires neither of those things. all AI metering does is count down stops from wide open. seriously. my FM2n has no idea what lens i'm using, what its maximum aperture is, or what aperture i've selected. yet it meters just fine. all it knows is how much light is hitting the meter, and how many stops i want to stop the lens down for the exposure.
Nikon cameras take their meter reading with the lens wide open. In order for the camera to evaluate the amount of light coming in, it has to know what the maximum aperture is. D and G lenses convey this info to the camera via the CPU, while AI/AIS lenses use a lens speed post sticking out of the back of the lens and require a lens speed indexing tab on the camera (pre-AI cameras use the "rabbit ears" sticking out of the side of the lens). Most cameras don't have this tab anymore, which is why you have to manually input the max aperture in the camera even on digital bodies that do meter with AI/AIs lenses.
no, you're quite confused here. there are posts that stick out of the back of lenses. they don't have anything to do with aperture; they tell some cameras (again, mostly the FA) the approximate focal length of the lens. the aperture information is communicated by the ridge of the rear of the aperture ring.
on digital bodies that
do have the tab that contacts this rings, you do not actually need to tell the camera either the focal length or the maximum aperture of the lens. the camera will function without either piece of information, and meter like an older AI camera (eg: an FM). your apertures will read ^f0, ^f1, ^f2, etc, counting down from wide open (^f0). giving the camera information about what that maximum aperture is, and what the focal length is, more or less only affects the meta-data (there's apparently some matrix metering voodoo going on as well).
on digital bodies that
do not have the tab, there's another prong that contacts the other portion of the ridge that tells the camera the lens is set to minimum aperture. the reason people tell you not to attach pre-AI lenses even to cheaper cameras is because this metering prong is still present. the d40's prong was spring-loaded and compressed into the body, so pre-AI lenses could be safely mounted.
Without an aperture follower tab, the camera has no way of knowing what aperture you've selected on the lens, thus it cannot give the appropriate shutter speed.
again, this is not true. both my d200 and my d700 meter just fine without lens information of any kind. in fact, if you have a camera that can meter with AI lenses, this is simple to test -- remove the lens information, blank it out. change the maximum aperture setting, in A mode. shutter speed will remain constant. all it really knows is:
1. how much light is hitting the meter,
2. how many stops you want the lens to stop down, and
3. the information you give it.
