Pentax K to Sony E adapter compatibility

Sony α is where most adapted Pentax K glass ends up, simply because the E-mount adapter ecosystem is the largest and the maths is friendly: the K-mount's 45.46 mm flange against Sony E's 18.0 mm leaves 27.46 mm of clearance, comfortably enough for a plain glassless ring to reach infinity with no corrective optics. On a full-frame body (α7 IV, α7R V, α7C II, α1, α9 III) you keep each lens's designed field of view; on an APS-C α (α6700, ZV-E10 II) you get a 1.5× crop that trims the corners.

What makes K-mount unusual is that both ends of the value spectrum are worth adapting. At the bottom, the SMC-M 50 f/1.7 is one of the cheapest competent fast normals in all of vintage photography, with the SMC-K 55 f/1.8, SMC-M 28 f/2.8, and SMC-M 135 f/3.5 rounding out a full prime kit for pocket money. At the top, the SMC FA Limited trio — the FA 31 f/1.8 Ltd, FA 43 f/1.9 Ltd, and FA 77 f/1.8 Ltd — are hand-finished aluminium-barrel primes that hold their value and have a rendering signature collectors chase. All of them mount on the same dumb ring; the only difference adapting is the price you paid for the glass.

The one thing to understand before buying is aperture control, because K-mount spans three behaviours. Pre-A manual lenses (the original SMC-K 55 f/1.8 and the SMC-M line) have a conventional aperture ring that drives the diaphragm directly — nothing to think about. A-series lenses (the SMC-A 50 f/1.4 and 28 f/2.8) add a green 'A' position that hands aperture to the camera through KA electrical contacts — but those contacts are dead on a dumb adapter, so you must take the ring off 'A' and set the f-stop by hand, exactly as you would a manual lens. Leave an A-lens locked on 'A' and it will stay stuck at minimum aperture (f/22). The later screw-drive and electronic KAF/KAF2 lenses lose autofocus entirely; focus is manual on everything.

The adapter itself is a commodity. K&F Concept, Urth, and Fotodiox all ship glassless K-to-E (PK-NEX) rings in the $20–40 band — our catalogue carries the K&F PK-NEX — and a Novoflex equivalent costs several times more for tighter tolerances that only matter if you shoot the fast Limiteds wide open daily. There is no glass in the ring, no electronics, and no aperture-actuating lever to worry about, since K lenses stop down from their own ring.

On the body side it is a fully manual experience: no autofocus, no electronic aperture, no aperture value in EXIF. Turn on stabilisation by entering the focal length by hand in Sony's SteadyShot menu, and lean on focus peaking plus magnify for critical focus. The reward is a characterful Pentax prime kit — whether you spent $40 on an SMC-M 50 or $700 on an FA 31 Limited — running on a current α sensor.