I found the answer on Boz site:
Joachim Hein... understood the roots of the exposure problems that
some Adaptall lenses have. These are all "slow" lenses with minimum
aperture setting of f/22 and an "AE" setting or f/32 and no "AE"
setting In the latter case, f/32 is converted to "AE"). "AE" is the
Tamron equivalent of the Pentax "A" setting. Looking in the second
column of the table above, we see that there is no contact pattern
for lenses with apertures ranging between f/4.5 or slower and f/22.
Tamron solves this by making the Adaptall mount indicate a maximum
aperture of f/4 and warning in the mount manual that the photographer
must watch the aperture read-out in the view-finder to ensure that
only values within the true aperture range are used.
I understand Tamron P/KA mount is a cripple A mount because its
basic mount contacts do not provide information on the maximum and
minimum aperture of the lens, which is necessary for matrix metering.
Is it possible to modify the mount (for a particular Tamron lens) to
provide the camera with matrix metering for that lens?
Is matrix metering possible with an A lens (Pentax or Tamron) that
has not a constant aperture?
Andre
