[davidmanze]

Sounds like RSA to me.

On the K3.

     Reasonable assumption (residual spherical abberation) but "no"! The lens AFs with the diaphragm fully open F2.8 always (auto diaphragm) ,normal!

 When you select F2.8-F5.0 and half press to activate AF, the point of focus (also indicated by the distance scale) is perfect on the subject, ie image perfectly focused! 

  Directly you select F5.6 or more and half press to activate AF the point on the distance scale changes to closer,and front focuses.

 

  You can check this in AF-C by holding half press to activate the AF continuously, and with the rear wheel choosing between F5 then F5.6 (obviously the diaphragm is always fully open) and it changes focus the position on the distance scale and front focuses.

 

 It's in fact it's a FW problem within the lens. It took me a while to realize what was happening, mostly I shoot at F5 close up, the error diminishes at distance, at infinity it's fine.

 

 The problem has completely disappeared now use the D750 with it's flawless AF. Next question, do I sell my Pentax gear?

 

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