Originally posted by blucoala Kenko or Tamron converter with SDM contacts for the PLM lens
You had me a little puzzled at first with the "SDM" piece. SDM is a lens motor type. But then I realized you meant the two power contacts within the mount throat. The power contacts on the K
AF2 and higher mounts were originally intended for power zoom on certain FA series lenses and were leveraged to supply power for in-lens focus and and aperture motors on later generation lenses. I am not sure what the proper name is, though I have seen them called both "power-zoom" and "power" contacts.
A pass-through type TC will pass through both power and data/control communication, but will not translate focal length or aperture. I don't believe that will be "fatal" though the following should be considered:
- SR may not be as effective as with a proper Pentax TC
- Recorded aperture in the EXIF metadata will not be correct
- Ditto the above for focal length
- Lens ID and other metadata should essentially be correct
- Viewfinder and LCD aperture and focal length displays will not be accurate
As far as the body is concerned, it has the PLM lens mounted directly. Likewise the lens is not aware that it is not talking to the body directly. The AF systems will drive the AF motor until focus is detected. Likewise the exposure system will detect attenuated light and expose properly in the same sense as if an ND filter were mounted. The actual f-number is not important for exposure purposes and is probably not even part of the conversation between lens and body.
Now the bad news...
As boriscleto noted above, PDAF performance will be compromised at the narrower relative aperture at full zoom and will likely fail completely as zoom approaches 420mm due to unavoidable prism 'black-out" at the f/9 maximum aperture for that focal length.
Translation: You can try it, but unless you are willing to use manual focus and/or live view, it is not going to work that well.
Steve