Whip me with a wet noodle if I haven't got this right, but I think it is because the lens has no means of registering which click stop the aperture ring is positioned at, except in the "A"-position. Even with the "A"-generation cameras, this was done mechanically with the aperture lever. Since the lever is gone, this functionality would require some other mechanism in the lenses. I have pondered many times why not use a electric resistor, for example, so that the conductivity changes for each click stop, and thus could be measured by the camera... -But then again it wouldn't help the older lenses much...
Jostein > From Boz's site I understand that the *istD mount lacks a stop-down coupler > on the body mount but does have the same six electric contacts as the KA > mount, plus a seventh electronic (digital) one. This digital contact > apparently communicates to the camera: 1) the focal legth of the lens; 2) > the lens to subject distance; 3) the lens dimensions; 4) MTF info; and 5) > the exact selected aperture. If this is the case - why doesn't the *istD > "know" the selected aperture when a F or FA lens has a manually set > aperture? I wouldn't have thought that the lack of a stop-down coupler on > this body would have mattered except when using A, M and K lenses. > > >
