Hi Pentaxians,
Thanks again for the comments.
Steve, the Sigma 70-300mm (just like the other AF lenses I tested the teleconverters with) works perfectly when mounted directly on the body, so this cannot be the reason for the problem. Also, you mentioned possible positioning/mounting problems or bad wiring. Well, the contacts seem to be at the right place and they look intact. The contacts are actually 2 pins connected with a copper spring so there are no solder joints, and the springs are not broken, either. (Once I find my multimeter I will check them for continuity but I guess if they were broken that would be visible and they would not keep the pins together, either.) Additionally, these teleconverters work properly with all KA lenses I tested them which also suggests that the 6 contacts are okay. Plus I have two teleconverters displaying the same problem and also found several reports about this issue (odd aperture displays when using KA mount teleconverters with autofocus lenses), so this suggests that this is not a specific case but something systematic. See for example:
https://www.pentaxforums.com/forums/58-troubleshooting-beginner-help/255683-v...#ixzz3Qg3eX8sC Strange aperture reading with 2x Teleconverter: Pentax SLR Talk Forum: Digital Photography Review
Then I read Bojidar Dimitrov’s excellent article about the KAF mount and everything became clear:
The KAF mount features the same six electric contacts as the KA, and thus remains fully compatible with the older mounts. However, in addition to those (electric) contacts, it features a seventh - electronic one. Using a yet-unknown serial protocol, the body uses this contact to communicate with a digital chip contained inside the lens housing. The chip provides at least the following information: - Focal length of the lens. - Lens-to-subject distance. - Exact selected aperture. - Lens dimensions. Features and Operation of the Kaf Mount
So it appears that the 6 contacts are needed to
control the aperture from the body but the 7th contact is necessary to
display it correctly.
And there came the idea: if my teleconverter has an extra contact (the Ricoh pin) that is exactly the same as the other 6 (2 pins connected with a copper spring) why not turn it into the 7th (electronic) contact? And I did! I documented the modding process and will post an article later.
And guess what: now the teleconverter works perfectly, it makes the body display the actual selected aperture with most autofocus lenses (Pentax FA series, Pentax DA series, Sigma DG series and Sigma DC series). (When I say correct aperture value I mean the native lens aperture without considering the two stops loss for the teleconverter).
Actually, there are 2 lenses that still make the body display incorrect apertures with the modified teleconverter: the Sigma 17-70mm F2.8-4 DC Macro OS HSM and the Pentax DA 18-135mm F3.5-5.6 ED AL [IF] DC WR. With these lenses, the displayed aperture values
flicker between 4 and 5.6 (wide open) and 22 and 32 (at minimum aperture).
Both lenses have an internal focus motor while the lenses displaying correct aperture values all have screw drive focus. Interestingly, I experience the same problem with my Vivitar 2XMC7 AF teleconverter which have 7 contacts but no SDM contacts, either. I still have no clue how the lack of the SDM contacts affect the aperture value display. This is getting weirder and weirder. Any ideas?
I would not worry about this as would not use a teleconverter with these lenses anyway but as mentioned earlier, I planned to convert the teleconverters to extension tubes by removing the glass, so it is a problem if they cannot be used with these 2 lenses. Any suggestions are welcome.
Thank you all.