> > William Robb wrote: > > When the lens is off A, the electrical contacts are disabled, and the > > lens has no electrical communication at all with the camera. > > It's not just AF cameras. The Super Program is the same way. > > Having switched to digital and no longer having any film camera bodies I > can't check this - but I could have sworn that F and FA lenses on an AF body > gave an aperture readout on the camera at all times (but A lenses didn't).
But that's because they communicate this information via the digital readout pin, not via the mechanical aperture coupler. As far as I know Pentax have never made a camera that used the maximum-aperture value (read from the A-mount contacts) and the aperture-delta value (from the aperture coupler) to calculate the actual lens aperture setting. There are varied reasons for this. For one thing, on the early A-mount caperas the only thing that read the aperture coupler was the metering circuitry - nothing else in the camera knew anything about the lens. Later on (by which time the cameras were incorporating digital logic) the lenses were communicating information over the digital signal pin, so there was no need to bother with reading the aperture coupler. An "A" lens off the "A" position is, in almost all cases, treated in just the same way as a K/M lens. Those older lenses don't have the contacts necessary to communicate maximum aperture.
