I've found that this depends on the exact mount. I have 2 Tamron Adaptall>P/KA mounts. One allows a maximum aperture of F/4, the other allows up to at least F/2.5 for my 135/2.5. Don't know if one is AD/2 and the other plain Adaptall or just why this is. Haven't taken the time to find out, the 135/2.5 is the only one of 9 Tamron Adaptall lenses that I have that I consider a truly "above average" lens.
Don > -----Original Message----- > From: Andre Langevin [mailto:[EMAIL PROTECTED] > Sent: Wednesday, October 19, 2005 3:41 PM > To: [email protected] > Subject: Re: Matrix metering with Tamron P/KA ? > > > 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 >
