Just spotted this thread and thought I'd relate my recent experience with this lens on a K5.
Bought the lens off ebay and it arrived in what looks like near mint condition (wonderful construction quality!), so very eager to see what results it turned in.
Initial shots looked fine, and nicely sharp, but they were taken between f/4.5 and f/8 and, as it turns out, the depth of focus was masking a fairly severe back-focusing issue, revealed on shots taken wide open.
Fine focus adjustment (+10, the maximum) was not enough to bring it in, and I was therefore worried that this would prove a difficult issue to resolve...
However, I found references to the possibility of applying auto-focus correction via a debug mode on the camera, and after a couple of hours yesterday morning now have the lens seemingly dialled in to my body, and all other lenses adjusted to cope with the extra 'correction' now applied. I can now auto-focus with some confidence.
If you have a PC (Windows) then the PK Tether method will allow entry to debug mode, if you are on Mac (as I am), then the means of access means placing specific files at the root level of a memory card and then turning the camera on with the card door open. See
Resurrecting Pentax Firmware Hacking (first three pages), and
Ricehigh's walkthrough process for tweaking auto-focus correction for more details.
Usual caveats apply though... debug mode is not meant to be accessible on K5 or any other pentax camera, so involves using reverse engineered methods of entry, and then exposes commands that could adversely affect other operation of the camera. If you do decide to try this then you do so at your own risk, and any error on your part could mean costly repair, or a new camera body.
For me it worked, and I now have a lens that I thought would be only marginally usable at f/5.6 or smaller, is now usable from wide-open.
Cheers
Rob
(aside: Pentax is very poorly supported in Thailand, verging on 'not at all'... at best it is 'difficult', and usually dependant upon some unofficial camera tech to get anything done... If there is anyone who knows different, I'd love to learn of reliable pentax support here)
Last edited by roblgs; 03-23-2014 at 06:09 PM.
Reason: Moved the aside to the bottom