I wonder whether it would be possible to mount Pentax lenses on a Sony NEX, using an adapter. Are such adapters already available? Would it be possible to have automatic focus?
Yes, they're available, from several sources. Two notable ones are:
- Rayqual, who make many similar adapters and other equipment [webiste (Japanese) and press release].
- Cosina, long-time photography manufacturer, recently acquired the Voigtländer brand [Stephen Gandy's page, not listed on Cosina's main site yet].
There are also versions available on eBay from various sellers, but I have no idea about reliability or quality of those. Rayqual and Cosina are both long-time manufacturers with pretty good reputations.
Autofocus will not be possible. AF systems aren't necessarily compatible to begin with, and the connection information isn't available to third parties. The best situation for most adapters is focus confirmation.
I have K&F Concept Lens Mount Adapter: http://www.amazon.com/Concept-Adapter-Pentax-E-Mount-NEX-VG10/dp/B00LECYTL4?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s01
It works perfectly and I like that the one I have for Nikon F Mount is from a different company (Kiwifoto) so its easy for me to identify which is which. It is all manual but then my Pentax lenses are all old manual ones anyways. I haven't seen any Pentax K adapters with electronics in them.
Yes, it is possible to mount and use Pentax K objectives with Sony E mount via adapter with following limitations:
- no autofocus and any function requiring electricity are usable
- no aperture coupling is possible, the aperture won't be opened for framing automatically and closed for exposure
- objectives without aperture ring will be have aperture closed unless an adapter features an aperture ring itself.
There are two kinds of mechanical adapters for PK. One that has an additional slider to operate the aperture on a lens without an aperture ring (eg the "Pentax- A SMC Zoom 1:4 28 – 135 mm" that was asked about in a comment), albeit in uncalibrated fashion.
The other style does not have this feature, making lenses with no aperture ring only usable wide open but eliminating a potential source of operator error (slider set the wrong way leads to aperture ring operation not having the expected effect).
The second cheapest choice of adapters (eg the K&F mentioned in another answer) is usually fine. There are even cheaper options, these sometimes will exhibit problems like too-loose fit (especially with third party lenses) or light leaks.