Why does the camera need to know the aperture settings of the lens?
The aperture setting is irrelevant, but the max aperture of the lens must be communicated to the camera.
Why does it need the max aperture? Couldn't it just work with whatever light reaches the AF sensor? The little I think know is that the AF sensor checks if the light reaching two pixels is equal (focus) or different (not focus), and that difference tells the distance to focus, and the sign of that difference tells the direction.
Even needing to know the max aperture, why not the camera:
1. Allow max aperture to be set, one more parameter in some menu page, rather than forcing electronics on every lenses for such small (and constant) piece of info?
2. Assume some value (e.g 5.6), at least it would work in most cases, whereas like this it never works...
Does the 70D dual-pixel also needs electronics on the lenses?